UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  ACRICULTURE 
AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


Citrus  Diseases  of  Florida  and  Cuba 

Compared  with  Those 

of  California 


By  HOWARD  S.  FAWCETT 


BULLETIN  No.  262 

Berkeley,    California 


California 
State  Printing  Office 
1915 
19334 


Benjamin  Ide  Wheeler,  President  of  the  University. 

EXPERIMENT   STATION  STAFF 

HEADS  of  divisions 

Thomas  Forsyth  Hunt,  Director. 

Eugene  W.  Hilgard,  Agricultural  Chemistry   (Emeritus). 

Edward  J.  Wickson,  Horticulture   (Emeritus). 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station  ;  riant  Breeding. 

Hubert  E.  Van  Norman,  Vice-Director ;  Dairy  Management. 

William  A.  Setchell,  Botany. 

Myer  E.  Jaffa,  Nutrition. 

Robert  H.  Loughridge,  Soil  Chemistry  and  Physics  (Emeritus). 

Charles  W.  Woodworth,  Entomology. 

Ralph  E.  Smith,  Plant  Pathology. 

J.  Eliot  Coit,  Citriculture. 

John  W.  Gilmore,  Agronomy. 

Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 

Frederic  T.  Bioletti,  Viticulture  and  Enology. 

Warren  T.  Clarke,  Agricultural  Extension. 

John   S.  Burd,  Agricultural  Chemistry. 

Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 

Clarence  M.  Haring,  Veterinary  Science  and  Bacteriology. 

Ernest  B.  Babcock,  Genetics. 

Gordon  H.  True,  Animal  Husbandry. 

James  T.  Barrett,  Plant  Pathology. 

Fritz  W.  Woll,  Animal  Nutrition. 

A.  V.  Stubenrauch,  Pomology. 

Walter  Mulford,  Forestry. 

W.  P.  Kelley,  Agricultural  Chemistry. 

H.  J.  Quayle,  Entomology. 

D.  T.  Mason,  Forestry. 

J.  B.  Davidson,  Rural  Engineering. 

P]lwood  Mead,  Rural  Institutions. 

William  G.  Hummel,  Agricultural  Education. 

Leon  M.  Davis,  Dairy  Industry. 

John  E.  Dougherty,  Poultry  Husbandry. 

Frank  Adams,  Irrigation  Practice. 

David  N.  Morgan,  Assistant  to  the  Director. 

Mrs.  D.  L.  Bunnell,  Librarian. 

DIVISION    OF    ENTOMOLOGY 

C.  W.  Woodworth  L.  J.  Nickels 

W.  B.  Herms  E.  O.  Essig 

E.  C.  Van  Dyke  S.  B.  Freeborn 

Geo.  A.  Coleman  E.  P.  Van  Duzee 

Geo.  P.  Gray  M.  R.  Miller 


CONTENTS. 


Page 

INTRODUCTION    . 153 

CITRUS   DISEASES    OF   FLORIDA   AND    CALIFORNIA   COMPARED 154 

Gum  Diseases 155 

Pythiacystis  Gummosis  (Pythiacystis  citrophthora) 155 

Botrytis    Gummosis    (Botrytis    cinerea) 158 

Mai  di  gomma  or  "Foot  Rot" 159 

Psorosis  or  California  Scaly  Bark 162 

Florida  Scaly  Bark  or  Nail-Head  Rust 164 

Diplodia  Gumming  (Diplodia  natalensis) 165 

Armillaria  Root  Rot    (Armillaria  mellea) 167 

Summary  of  Gum  Diseases 168 

Diseases  op  Twigs,,  Leaves,  and  Fruit,  Except  Rots 169 

Citrus   Canker    (Pseudomonas   citri) 169 

Melanose   (Phomopsis  citri) 174 

Verrucosis   or   Scab    (Cladosporium   citri) 176 

Wither-tip   of  Limes    (Gloeosporium  Limetticolum) __  178 

Wither-tip    of    Citrus    Trees    Other    Than    Limes    (Colletotrichum    ghro- 

sporioides)     180 

Exanthema   or   Die    Back 182 

Mottled   Leaf 184 

Chlorosis 185 

Citrus  Blight 185 

Citrus    Fruit   Rots 186 

Blue  and  Green  Molds   (Penicillium  italicum  and  P.  digitatum) 187 

Stem  End  Rot   (Phomopsis  citri) 188 

Anthracnose    (Colletotrichum  gloeosporioides) 189 

Black  Rot  of  Oranges   (Alternaria  citri) 189 

Diplodia   Rot    (Diplodia   natalensis) 189 

Brown  Rot   (Pythiacystis  citrophthora) 190 

Cottony  Rot   (Sclerotinia  libertiana) 190 

Grey  Mold   (Botrytis  cinerea) 190 

Black  Pit   (Pseudomonas  citriputeale) 190 

Brown  Spot 191 

Peteca    __ _ 191 

Green  Spot  ___ 192 

THE   PRINCIPAL   INSECT   PESTS   OF   FLORIDA  AND  CALIFORNIA 

COMPARED 193 

Whiteflies    193 

The  Purple  Scale 196 

Other  Scale  Insects 196 

Red  Spider  and  Mites 197 


CITRUS  DISEASES  IN  CUBA  AND  THE  ISLE  OF  PINES 198 

Introduction . 198 

Insect  Pests 198 

Blue-Green  Beetles 198 

Leaf-Cutting   Ant    199 

Whiteflies  __ _ 199 

Scale  Insects  and  Mites 199 

Diseases  Other  Than  Insect  Pests 200 

Gum   Diseases 200 

Scab    (Cladosporium  citri) 201 

Wither-tip  of  Limes    (Gloeosporium  Limetticolum) ■_: 201 

Diplodia  Rot   (Diplodia  natalensis) 202 

Leaf  Spot  Due  to  an  Alga  (Mycoidea  parasitica) _i 202 

Dark  Greasy  Spot   (Black  Melanose) 202 

TABLE    COMPARING   THE    CITRUS   INSECT    PESTS    OF   CALIFORNIA, 

FLORIDA    AND    CUBA 203 

TABLE  COMPARING  THE  CITRUS  DISEASES  OF  CALIFORNIA,   FLORIDA 

AND   CUBA 207 


CITRUS  DISEASES  OE  FLORIDA  AND  CUBA  COMPARED  WITH  TNOSE  OF 

CALIFORNIA* 

By  Howard  S.  Fawcett. 


INTRODUCTION. 

During  the  months  of  January  and  February,  1914,  the  writer  made 
a  trip  to  Florida  and  Cuba,  for  the  purpose  of  collecting  bud  wood  of 
as  many  varieties  and  strains  of  citrus  trees  as  possible,  and  to  collect 
information  in  regard  to  citrus  conditions,  especially  in  regard  to 
citrus  diseases.  Although  much  had  been  written  in  regard  to  citrus 
diseases  by  workers  in  California,  Florida,  and  Cuba,  some  uncertainty 
still  existed  as  to  the  identity  of  some  of  the  important  diseases  in  these 
places.  In  some  cases,  different  names  were  apparently  being  used  for 
what  was  thought  to  be  the  same  disease,  and  in  other  cases,  the  same 
name  was  being  employed  for  what  appeared  to  be  entirely  different 
diseases.  It  also  seemed  that  certain  important  diseases  of  one  state 
were  either  absent  or  of  minor  importance  in  the  others.  This  resulted 
in  some  cases  in  unnecessary  uneasiness  and  confusion  to  citrus  growers, 
especially  in  cases  where  articles  or  bulletins  written  expressly  for  con- 
ditions in  one  state,  were  copied  by  the  agricultural  papers  of  the 
others. 

So  far  as  known,  no  one  acquainted  with  citrus  disease  conditions  in 
both  Florida  and  California,  had  ever  made  a  careful  comparative  study 
of  them.  It,  therefore,  seemed  advisable  that  this  comparative  study 
be  made  so  as  to  aid  the  growers  and  horticultural  officers  to  know  which 
diseases  were  different  and  which  were  identical,  and  to  enable  them  to 
more  easily  detect  at  once  a  new  disease,  should  it  by  any  unsuspected 
means  get  into  the  State.  As  the  writer  had  been  connected  with  the 
Florida  Agricultural  Experiment  Station,  working  on  citrus  diseases 
for  six  years  prior  to  his  work  in  California,  it  was  thought  that  he  was 
in  a  position  to  make  this  comparative  study. 

Because  of  the  recent  discovery  of  the  causal  agents  of  some  of  the 
gum  diseases  in  California,  a  special  study  and  observation  of  these 
were  made  during  the  trip.  The  most  interesting  result  of  this  special 
work  was  that  the  brown  rot  fungus,  Pythiacystis  citrophthora,  which 
had  been  proved  to  be  the  causal  agent  in  the  most  common  form  of 
gummosis  in  California,  was  isolated  from  diseased  specimens  in  Flor- 
ida, in  Cuba,  and  in  the  Isle  of  Pines. 

In  this  bulletin  a  comparison  of  disease  conditions  between  Florida 
and  California  will  first  be  considered,  and  afterwards  the  conditions  in 


*Paper   No.    10,    Citrus   Experiment   Station,    College   of   Agriculture,   University   of 
California,  Riverside,  California. 


154  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION. 

Cuba  will  be  discussed  separately  in  comparison  with  the  first  two 
places. 

The  writer  wishes  to  acknowledge  the  aid  rendered  in  connection  with 
this  study  by  P.  H.  Rolfs,  H.  E.  Stevens,  E.  W.  Berger  and  J.  R.  Wat- 
son of  the  Florida  Agricultural  Experiment  Station;  by  F.  S.  Earle, 
Robert  Luaces,  H.  0.  Neville,  A.  Beatley,  and  H.  A.  Van  Herman  of 
Cuba,  and  by  W.  T.  Home  of  the  University  of  California.  The 
author  is  also  indebted  to  H.  J.  Webber  and  H.  J.  Quayle  for  helpful 
suggestions  in  regard  to  certain  parts  of  the  manuscript. 

CITRUS  DISEASES  OF  FLORIDA  AND  CALIFORNIA  COMPARED 

One  of  the  most  striking  differences  in  the  citrus  conditions  in  the 
two  states,  is  that  as  a  rule  the  most  important  diseases  in  Florida  are 
either  absent  or  of  small  importance  in  California  and  that  the  most 
important  diseases  in  California  are  absent  or  of  less  importance  in 
Florida.  This  is  probably  due  largely  to  the  influence  of  differences  in 
soil,  climate  or  other  environmental  conditions  in  the  two  states.  In 
Florida,  the  principal  citrus  regions  lie  between  25  and  30  degrees  north 
latitude,  while  in  California  they  lie  between  32J  and  39J  north  latitude, 
or  the  same  latitude  as  the  territory  between  Washington,  D.  C,  and 
Charleston,  South  Carolina.  Florida  has  a  rainfall  of  40  to  60  inches 
mostly  during  the  late  spring  and  summer,  while  the  citrus  regions  of 
California  have  a  rainfall  of  10  to  25  inches  mostly  during  the  late  fall 
and  winter. 

The  most  widely  distributed  and  perhaps  most  serious  citrus  diseases 
in  Florida  (not  including  the  extremely  serious  and  recently  introduced 
citrus  canker  which  is  discussed  later)  are  melanose  (with  its  associated 
disease,  stem  end  rot),  exanthema,  wither-tip,  and  foot  rot.  There  are 
others  such  as  scab,  nail-head  rust,  psorosis,  Diplodia  gumming,  blight, 
and  mottled  leaf,  which  locally  or  in  abnormal  seasons  may  assume  as 
great  or  even  greater  importance  than  some  of  those  first  mentioned, 
but  which  generally  are  less  serious  when  the  entire  state  is  considered. 

In  California,  a  corresponding  list  of  the  most  important  would  per- 
haps be  the  brown  rot  gummosis  (with  its  associated  disease,  brown  rot 
of  the  fruit),  mottled  leaf,  psorosis  (also  called  California  scaly  bark), 
and  possibly  Armillaria  root  rot  (although  the  last  is  only  serious 
locally),  with  foot  rot,  wither-tip,  exanthema,  Botrytis  gummosis  of 
lemons,  Botrytis  and  Sclerotinia  rot  of  lemon  fruit  possibly  in  the  list 
of  those  of  less  importance,  except  locally  or  in  abnormal  seasons.  It 
will  be  seen  that  none  of  the  list  of  first  importance  in  Florida  appear 
in  the  corresponding  list  in  California.  Three  of  the  list  of  most  im- 
portance in  Florida,  exanthema,  wither-tip,  and  foot  rot,  occur  in  the 
list   of   secondary   importance   in   California,    and   two,    psorosis   and 


[Bulletin  262]  citrus  diseases.  155 

mottled  leaf,  of  the  list  of  most  important  in  California,  occur  in  the 
list  of  secondary  importance  in  Florida. 

As  far  as  possible,  diseases  producing  somewhat  similar  effects  or 
found  on  the  same  parts  of  the  citrus  tree,  will  be  taken  up  in  groups : 
first  the  gum  diseases  or  diseases  principally  of  the  trunk  and  larger 
branches;  second,  the  twig,  leaf  and  fruit  diseases  other  than  the  rots; 
and  third,  the  fruit  rots.  Some  of  the  diseases  that  are  rare  or  of  little 
importance  will  be  found  only  in  the  tabulated  list  at  the  end  of  the 
bulletin. 

GUM  DISEASES 

Considerable  confusion  and  misunderstanding  has  arisen  in  regard  to 
gum  diseases  of  citrus  trees  both  in  Florida  and  California  because: 
first,  there  are  at  least  seven  apparently  different  and  distinct  troubles 
in  which  gum  oozes  out  from  the  trunk  or  larger  limbs  of  citrus  trees ; 
and  because,  second,  some  of  these  troubles  have  come  to  be  known  by 
growers  and  others  under  one  name  in  California  and  under  an  entirely 
different  name  in  Florida,  the  same  name  in  at  least  one  case,  scaly 
bark,  having  been  used  unfortunately,  for  different  troubles  in  the  two 
places;  and  because,  third,  some  forms  of  gumming  appear  to  be  com- 
mon in  one  place,  and  are  rare  or  not  found  at  all  in  the  other. 

PYTHIACYSTIS  GUMMOSIS  (Pythiacystis  citrophthora) . 

The  Pythiacystis  or  brown  rot  gummosis  with  its  associated  rot  of 
fruit  due  to  the  same  fungus,  probably  heads  the  list  of  fungous  diseases 
of  citrus  trees  in  California.  In  1913, 1  the  writer  first  proved  that  the 
brown  rot  fungus  (Pythiacystis  citrophthora  was  the  causal  agent  in  the 
occurrence  of  this  most  common  form  of  gum  disease  in  California. 
Another  form  of  gummosis  formerly  considered  the  same  as  the  above, 
was  shown  to  be  due  in  part  to  another  fungus,  Botrytis  cinerea.  These 
two  forms  are  included  in  the  "Gummosis  or  Lemon  Gum  Disease," 
described  by  R.  E.  Smith  and  0.  Butler  in  Bulletin  218  of  the  California 
Agricultural  Experiment  Station. 

The  Pythiacystis  or  brown  rot  gummosis  is  characterized,  especially 
on  lemon  trees,  by  killing  of  areas  of  bark  on  the  trunk  through  to  the 
wood  and  by  the  exudation  of  large  quantities  of  gum.  The  infection 
usually  starts  at  the  base  or  just  above  the  crown  roots  and  works  rap- 
idly upward  and  sidewise  on  the  tree.  The  bark  remains  firm  (not 
soft)  as  it  is  killed  without  evidence  of  fungus,  and  as  the  bark  dries 
it  shrinks  and  cracks  (Fig.  la)  longitudinally.  The  progress  of  the 
disease  appears  to  be  most  rapid  during  the  late  spring  and  summer 
in  California  and  is  checked  in  the  fall.  The  lemon  bark  is  most 
seriously  affected.     On  the  bark  of  old  orange  trees  the  progress  of  the 

3Monthly  Bulletin,   State  Commission  of  Horticulture,  Volume  II,  pages  601  to  617- 


156 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 


Fig.  1.  Pythiacystis  or  brown  rot  gummosis  on  lemon  trunks.  (a) 
Naturally  occurring  case  on  a  low  budded  15 -year-old  tree  now  beyond 
recovery.  (&)  Tree  eighteen  years  old  showing  result  of  inoculation 
eleven  months  previous  with  pure  culture  of  Pythiacystis  fungus  at  X. 
The  white  line  is  drawn  around  the  advancing  margin  of  the  killed  area 
of  bark,  (c)  Tree  eighteen  years  old  with  bark  cut  away  to  show  result 
of  inoculation  eight  months  previous  with  pure  culture  of  Pythiacystis" 
fungus  at  X.  Bark  cut  away  more  than  is  necessary  for  treatment. 
(d)  Same  tree  nine  months  later  after  being  treated  with  Bordeaux  paste 
showing  band  of  new  bark  around  edges  of  area  cut  out. 


[Bulletin  262]  citrus  diseases.  157 

disease  is  often  soon  arrested  and  the  edges  of  the  killed  areas  heal  and 
the  bark  breaks  away  much  as  in  the  case  of  mal  di  gomma  in  Florida. 
This  form  of  gummosis  was  also  found  present  to  some  extent  in 
Florida  and  cultures  of  Pythiacystis  citrophthora  were  obtained  from 
diseased  bark  of  a  grapefruit  tree  that  had  apparently  been  affected 
with  mal  di  gomma  or  foot  rot.  The  bark  was  killed  entirely  through 
to  the  wood  and  the  area  had  progressed  upward  further  than  is  usual 
with  most  typical  cases  of  foot  rot  in  Florida  (Fig.  5).  The  killed  bark 
had  the  same  color  and  odor  as  that  noticed  in  connection  with  the  same 
disease  in  California. 

In  Cuba  and  in  the  Isle  of  Pines  where  the  soil  is  heavier  than  in 
Florida,  this  form  of  gummosis  was  seen  in  nearly  every  locality  visited. 
The  Pythiacystis  fungus  was  obtained  in  cultures  from  diseased  bark  of 
grapefruit  trees  at  Santiago  de  las  Vegas,  Cuba,  and  from  a  tangelo 
tree  at  Santa  Fe,  Isle  of  Pines,  but  no  fruits  affected  with  the  fungus 
were  found  in  Florida,  in  Cuba,  or  in  the  Isle  of  Pines.  The  cultures 
from  all  of  these  places,  on  being  placed  on  lemon  fruits  in  the  labora- 
tory, produced  typical  brown  rot  identical  with  that  produced  by 
cultures  isolated  from  California  specimens  of  gummosis.  As  far  as  the 
writer  is  aware,  this  was  the  first  time  that  Pythiacystis  citrophthora 
has  been  isolated  in  cultures  from  any  of  these  localities.  In  1912, 
Mr.  H.  J.  Ramsey  in  correspondence  with  the  writer,  stated  that  he  had 
found  oranges  near  Miami,  Florida,  which,  judging  from  the  odor, 
appeared  to  be  affected  with  the  brown  rot  as  occurring  in  California, 
but  the  fungus  was  not  identified  at  that  time. 

The  Pythiacystis  fungus  which  lives  in  the  soil  usually  infects  the 
bark  at  or  below  the  surface  of  the  soil.  The  causal  conditions  for 
infection  are:  Improper  drainage,  continuous,  excessive  irrigation,  let- 
ting the  water  stand  too  near  the  trunk,  deep  planting,  or  allowing  the 
soil  to  wash  in  about  the  trunks  of  the  trees,  and  injuries  to  the  bark  at 
the  base  of  the  tree  in  digging  about  it  in  wet  weather.  This  applies 
particularly  to  heavier  soils  in  California  in  which  the  fungus  appears 
to  find  more  congenial  conditions  for  development  than  in  lighter  soils. 
Inoculations  have  shown  that  sour  orange  bark  is  quite  resistant  to  the 
attack  of  the  Pythiacystis  fungus,  while  lemon  bark  is  very  susceptible, 
and  sweet  orange  bark  stands  between  sour  orange  and  lemon  in  its 
susceptibility.  The  disease  can  therefore  be  largely  prevented  on  lemon 
trees  by  budding  high  on  sour  orange  stocks  in  order  to  get  the  suscept- 
ible lemon  bark  away  from  the  soil.  The  method  of  treatment  for 
Pythiacystis  gummosis  that  has  been  worked  out  in  California  by  the 
writer  in  co-operation  with  the  growers2  is  as  follows :  The  extent  to 
which  the  bark  is  killed  to  the  wood  in  the  vicinity  of  the  exuding  gum 


2Fawcett,    California    State    Commission    of    Horticulture,    2:    601-617,    1913;    and 
Prizer,  California  State  Commission  of  Horticulture,  4:  7-19,  1915. 

2—19334 


158  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 

is  first  ascertained  by  scraping  slightly.  Then  the  bark  is  cut  with  a 
heavy  knife  all  around  the  area  about  one-half  to  one  inch  into  the  live, 
unstained  bark  and  a  little  farther  at  the  top  than  at  the  sides,  as  the 
fungus  usually  extends  faster  upwards  than  in  other  directions.  Then 
this  entire  area  of  bark  that  is  killed  to  the  wood  is  dissected  out  and 
the  wound  and  the  entire  trunk  is  treated  with  Bordeaux  paste.  After 
the  area  is  seen  to  be  healing  at  the  margins  of  the  cut-out  area,  the 
exposed  wood  is  then  covered  with  asphalt  paint,  shellac  dressing  or 
other  good  covering.  The  formula  for  Bordeaux  paste  is :  One  pound 
of  bluestone  (copper  sulphate)  dissolved  in  three  quarts  of  water  in  a 
wooden,  earthen  or  glass  vessel ;  and  two  pounds  unslaked  lime  slaked  in 
three  quarts  of  water  in  a  separate  vessel  of  any  kind.  When  the  lime 
is  cool,  stir  the  two  together,  making  a  slightly  pasty  light  blue  mixture. 
The  possible  relation  of  Pythiacystis  to  mal  di  gomma  will  be  briefly 
discussed  later  under  that  subject. 

BOTRYTIS  GUMMOSIS  (Botrytis  cinerea)* 

This  differs  from  the  brown  rot  gummosis  in  killing  the  outer  part  of 
the  bark  on  lemon  trees  much  in  advance  of  the  inner.  The  writer  has 
never  seen  it  on  trees  younger  than  ten  years  old.  An  area  of  bark, 
usually  small,  will  be  found  killed  through  to  the  wood,  but  sur- 
rounding this  a  much  larger  area  where  the  outer  bark  has  been  killed 
and  the  inner  bark  is  still  alive. 

The  Botrytis  gummosis  was  not  found  on  lemon  trees  in  Florida. 
The  few  lemon  trees  that  are  left  in  Florida  are  usually  neglected  and 
are  found  only  as  dooryard  trees  or  as  chance  trees  left  in  an  orange  or 
grapefruit  grove.  Nearly  every  lemon  tree  seen  was  gumming  more  or 
less  at  the  trunk,  but  no  Botrytis  or  Pythiacystis  was  obtained  in 
cultures  from  any  of  these.  Pliomopsis  citri  or  Diplodia  natalensis 
appeared  to  be  the  most  common  fungi  in  the  diseased  bark.  The  bark 
was  usually  killed  through  to  the  wood  only  in  small  areas.  The  form 
of  gummosis  was  more  like  a  certain  scaliness  of  the  bark  on  old  lemon 
trees  in  California  where  the  outer  bark  dies  and  cracks  up  in  long 
strips,  somewhat  like  the  bark  on  a  shell  bark  hickory  tree  of  the  eastern 
states.  (Fig.  2.)  This  usually  first  appears  on  lemon  trees  about  15  to 
20  years  old  or  older  in  California.  The  Eureka  variety  seems  to  show 
it  earlier  than  the  Lisbon.  It  is  not  yet  known  certainly  whether  this 
is  a  condition  through  which  old  lemon  trees  naturally  pass  or  whether 
a  fungus  which  attacks  the  outer  bark  may  be  a  causal  agent.  When 
the  bark  is  first  cracking  and  loosening,  the  tree  usually  appears  ' '  out  of 
condition."  The  old  cracked  bark  tends  to  encourage  gum  disease, 
especially  the  Botrytis  form. 


*The  specific  name  of  this  fungus  was  tentatively  determined  by  G.  E.  Atkinson  of 
Cornell  University. 


[Bulletin  262] 


CITRUS   DISEASES. 


159 


In  the  treatment  of  Botrytis  gummosis,  areas  that  are  dead  to  the 
wood  are  cut  out  as  in  the  Pythiacystis  form,  but  beyond  this  the  outer 
diseased  bark  only  is  scraped  off,  leaving  the  inner  back  attached,  and 
the  wound  treated  with  Bordeaux  paste  or  other  fungicides  that  are 
not  injurious  to  the  bark. 


Fig.  2.  Showing  the  cracking  and  scal- 
ing (not  Psorosis)  of  bark  on  old  lemon 
tree  trunks  in  California.  Note  the  free- 
dom from  cracking  on  the  orange  stock 
below  the  bud  union. 

MAL   Dl    GOMMA   OR    FOOT    ROT. 

Foot  rot  is  very  common  in  Florida,  especially  on  old  seedling  trees. 
This  form  of  gum  disease  is  scarcely  distinguishable  from  the  brown  rot 
gummosis,  except  that  in  its  most  common  form  in  both  Florida  and 
California  on  old  seedling  orange  trees,  it  scarcely  ever  extends  so  far 
above  the  ground.  Gum  exudes  on  the  trunk  of  the  tree  above  the  soil. 
The  inner  bark  and  finally  the  wood  underneath  becomes  rotten  and 


160 


UNIVERSITY    OF    CALIFORNIA EXPERIMENT   STATION. 


often  has  a  very  disagreeable,  fetid  odor.  The  bark  dies  and  breaks 
away  in  patches,  leaving  bare,  dead  areas  which  spread  in  all  directions, 
mostly  down  on  the  main  crown  roots  and  laterally  around  the  trunk. 
(Figs.  3  and  4.)  Trees  thus  affected  bear  heavily  and  the  leaves  become 
yellow. 


Fig.  3.  Mai  di  gomma  or  foot  rot.  (a)  On  forty-year-old  seedling  orange  trees 
(California).  White  lines  indicate  the  boundaries  of  the  killed  bark.  (6)  Earth  dug 
away  and  the  affected  bark  and  roots  being  removed  before  applying  Bordeaux  paste. 


[Bulletin  262] 


CITRUS   DISEASES. 


161 


The  following  conditions  appear  to  favor  the  development  of  foot  rot 
in  Florida,  although  cases  may  be  found  under  all  conditions :  Improper 
drainage,  too  close  planting,  continuous  excessive  irrigation,  deep  plant- 
ing. These,  as  will  be  noticed,  are  the  same  conditions  as  those  that 
favor  the  occurrence  of  brown  rot  gummosis  in  California.  Fortu- 
nately, foot  rot  as  well  as  gummosis  may  be  largely  prevented  by  budding 
nursery  trees  to  sour  orange  roots.  This  has  been  done  for  many  years 
in  Florida  and  has  controlled  the  disease  perfectly,  even  in  cases  in 
which  the  sour  orange  roots  were  used  as  resets  in  badly  infested 
orchards.     H.  H.  Hume3  says  that  in  order  of  foot  rot  resistance  differ- 


FlG.    4. 


Mai   di   gomma   or   foot   rot  on   sweet   seedling   orange   tree   in   Florida. 
(Swingle  and  Webber.) 


ent  citrus  stocks  stand  about  as  follows:  (1)  sour  oranges,  (2)  pomelo, 
(3)  rough  lemon,  (4)  lemon,  (5)  sweet  orange.  The  first  three,  he 
states,  may  be  roughly  classed  as  decidedly  resistant.  The  last  two  as 
much  subject  to  the  disease.  The  writer's  observations  in  Florida  and 
Cuba  would  lead  him  to  the  conclusion  that  certain  strains  of  pomelo 
stock  were  nearly  as  susceptible  to  mal  di  gomma  as  the  sweet  orange. 

Many  foot  rot  trees  have  been  cured  in  Florida  by  digging  away  the 
earth,  thus  exposing  and  drying  out  the  crown  roots,  and  cutting  away 
and  cleaning  out  the  decayed  areas  and  disinfecting  them.  A  number 
of  good  disinfectants  have  been  used  with  about  the  same  success.  Some 
of  these  are  crude  carbolic  acid  and  water  1  to  3  parts,  Avenarius  car- 
bolineum  one  gallon  to  one  gallon  of  water  in  which  one  pound  of  whale- 

3Florida  Agricultural  Experiment  Station  Bulletin  53. 


162 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 


oil  soap  has  been  dissolved,  or  Bordeaux  paste  applied  like  whitewash 
(see  formula  for  Bordeaux  paste  under  ' * Py thiacystis  Gummosis"). 
Cleaning  out  and  drying  out  the  crown  roots  seem  to  be  more  important 
than  the  disinfecting. 

Sometimes  the  disease  extends  farther  up  on  the  trunk  than  usual  and 
in  that  form  appears  to  be  typical  with  the  brown  rot  gummosis.  It 
would  seem  from  our  present  knowledge  that  either  there  are  two  dis- 
eases that  have  been  going  under  the  name  of  mal  di  gomma  (foot  rot) 
in  Florida  or  that  mal  di  gomma  is  only  another  manifestation  of 
Pythiacystis  or  brown  rot  gummosis.4     (Fig.  5.) 


Fig.   5.     Mal  di  gomma  on  seedling  grapefruit  tree  in  Florida.     Cultures 
of  Pythiacystis  citrophthora  were  obtained  from  bark  taken  out  at  X. 

PSOROSIS   OR    CALIFORNIA   SCALY    BARK. 

The  term  California  scaly  bark  is  used  here  because  of  the  fact  that  in 
Florida  the  term  scaly  bark  is  used  for  a  different  disease.  The  unfor- 
tunate use  of  the  same  name  for  two  distinct  troubles  seems  to  have 
come  about  because  the  names  were  suggested  by  a  certain  scaling  and 
flaking  of  the  bark  that  is  common  to  the  two  diseases. 

It  may  be  said  with  certainty  that  what  has  been  known  as  Psorosis5 
of  orange  trees  in  Florida  and  what  has  been  known  as  scaly  bark6  of 
orange  trees  in  California  are  one  and  the  same  disease.  All  the 
symptoms  and  effects  are  the  same  in  both  places. 


'Phytopathology  5,  66-67,  1915. 

BSwingle  and  Webber,  Division  of  Vegetable  Physiology  and  Pathology,  Bulletin  8, 
p.  30. 

6Smith  and  Butler,  California  Agricultural  Experiment  Station,  Bulletin  200. 


[Bulletin  262]  citrus  diseases.  163 

The  disease  is  more  common  and  more  widely  distributed  in  Cali- 
fornia than  in  Florida.  In  most  places  in  California  the  Navel  orange 
appears  to  be  slightly  more  subject  to  it  than  the  Valencia,  and  only  a 
few  cases  of  what  appears  to  be  the  same  disease  have  been  seen  on 
grapefruit  trees  here.  The  same  or  a  similar  trouble  is  fairly  common 
on  grapefruit  and  tangerine  trees  in  Florida.  Only  a  few  trees  affected 
with  this  disease  were  seen  in  Cuba. 


ij^r 


n 


Fig.  6.  Psorosis  on  seedling  orange 
trunk  in  California,  showing  scales  of 
bark  peeling  off. 

Psorosis  manifests  itself  by  the  outer  bark  being  broken  into  scales 
and  pushed  up  over  areas  varying  in  size  from  an  inch  to  a  foot  or 
more  in  diameter.  It  begins  in  a  very  small  area  in  which  simply  the 
outer  bark  dies,  hardens  and  is  pushed  off,  apparently  by  the  formation 
of  new  tissue  underneath.  This  goes  on  from  year  to  year,  sometimes 
very  slowly,  only  an  inch  or  so  each  year  for  several  years,  before  an 
area  of  any  importance  is  formed  (Fig.  6). 

Briefly  stated,  Psorosis  or  scaly  bark  of  orange  trees  may  be  said  to 
have  three  stages  in  its  development. 


164  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION. 

1.  The  beginning  areas,  where  a  small  patch  of  outer  bark  is  crack- 
ing up  and  is  being  pushed  off  in  scales  by  the  growth  of  new  bark 
underneath. 

2.  The  enlargement  and  spread  of  this  first  stage,  until  the  areas 
surround  the  trunk  or  limbs  without  yet  killing  any  wood. 

3.  A  further  development  of  the  second  stage,  until  patches  of  bark 
are  killed  through  and  the  wood  beneath  begins  to  die  for  some  distance 
inward.  This  stage  may  require  from  five  to  ten  years'  time  from  the 
appearance  of  the  first  stage. 

The  first  stage  may  be  cured  by  cutting  out  affected  bark  and  treating 
with  a  good  fungicide,  such  as  was  mentioned  under  mal  di  gomma. 
The  second  stage,  if  on  the  trunk  or  largest  limbs,  may  be  checked  and 
possibly  cured  by  carefully  scraping  off  dead  bark  without  cutting 
through  the  live  bark  underneath  and  treating.  As  far  as  known  to  the 
writer,  there  is  no  satisfactory  remedy  for  trees  in  the  third  stage. 
Since  the  disease  is  so  slow  in  its  progress  and  its  spread,  advancing 
only  a  few  inches  each  year  and  often  taking  five  or  ten  years  before 
doing  evident  damage,  one  or  two  thorough  tree-to-tree  inspections  of 
an  orange  grove  each  year  should  be  sufficient  to  detect  most  cases  in  the 
first  stage,  when  they  are  easily  cured. 

FLORIDA    SCALY    BARK    OR   "NAIL-HEAD    RUST." 

This  disease  is  entirely  distinct  from  the  California  scaly  bark  which 
is  treated  of  under  a  previous  heading.  It,  as  well  as  the  Diplodia  gum- 
ming, is  not  known  to  exist  in  California.  It  is  also  a  very  destructive 
disease,  because  in  its  severe  form  it  not  only  covers  the  tree  with 
ruptured  bark  from  the  trunk  to  the  smallest  branches  and  twigs,  but 
also  spots  the  fruits  and  causes  them  to  drop  just  before  maturity. 

It  is  extremely  important,  therefore,  that  neither  this  disease  nor  the 
Diplodia  gumming  be  allowed  in  any  way  to  get  into  this  State.  The 
disease  was  fully  described  and  illustrated  by  the  writer  in  Bulletin 
No.  106  of  the  Florida  Agricultural  Experiment  Station.  It  is  known 
in  Florida  and  distinguished  from  the  California  scaly  bark  by  the 
following  characteristics : 

1.  On  the  smooth  bark  of  branches  6  to  9  months  old  and  older  will 
be  found  more  or  less  round  or  oval  spots  one-sixth  to  one-half  inch 
broad,  raised  above  the  surface,  rusty  in  color,  with  well-marked  edges 
(Fig.  7).  As  the  spots  grow  older  the  bark  becomes  glazed,  then  brittle, 
shows  cracks  running  lengthwise,  and  at  last  breaks  into  small  flakes  and 
scales.  In  severe  cases,  these  spots  increase  in  number  till  they  join 
together,  forming  large  patches  of  rusty  or  reddish-brown  scaly  and 
scabby  bark,  accompanied  by  small  drops  of  gum. 

2.  On  the  larger  limbs,  and  sometimes  on  the  trunk,  will  be  seen 
patches  of  rough  scaly  bark,  the  small  pieces  of  which  break  off  as  the 


[Bulletin  262]  citrus  diseases.  165 

hand  is  passed  over  them.  In  this  stage  it  is  hard  to  distinguish  it 
from  the  psorosis  or  California  scaly  bark.  (In  some  cases  this  scaliness 
of  the  old  bark  may  be  absent.) 

3.  On  the  fruit  will  usually  be  found  brown  sunken  spots,  some  of 
which  are  apt  to  be  in  the  form  of  sunken  rings.  These  spots  are  at  first 
yellowish  to  reddish  brown  on  the  green  fruit,  and  finally  become  dark 
and  sunken.  In  ringed  spots  the  rings  first  become  sunken  with  a 
higher  part  inside.  This  central  part  afterwards  sinks  in,  and  the  whole 
area  inside  the  ring  becomes  dark.  The  spots  vary  in  breadth  from 
one-fifth  to  one-half  inch. 

The  initial  spots  on  small  branches  have  been  produced  by  the  writer 
in  Florida  by  inoculation  with  a  fungus  known  as  Cladosporium  her- 
barum  var.  citricolum.     There  seemed  to  be  other  unknown  factors  or 


Fig  7.  Nail-head  rust  or  Florida  scaly  bark  spots  on  orange  branch  about  one  year 
old.  Natural  size.  The  surface  of  these  spots,  unlike  those  of  citrus  canker,  are  hard 
and  glazed  and  rarely  occur  on  grapefruit. 

organisms  that  entered  into  the  cause  for  the  later  stages  in  the  develop- 
ment of  the  diseased  areas. 

DIPLOPIA   GUMMING  (Diplodia  natalensis) . 

This  form  of  gumming  is  not  known  to  exist  in  California.  It  is 
fairly  common  in  southern  Florida  and  in  Cuba.  The  writer  found  a 
Diplodia  fungus  present  in  the  discolored  bark  and  wood  of  limbs 
affected  with  this  form  of  gumming.  When  this  fungus  was  isolated 
in  pure  cultures  and  inserted  into  cuts  in  the  bark  of  healthy  trees  a 
copious  flow  of  gum  with  killing  of  bark  and  wood  tissue  always 
resulted  (Fig.  8).  The  fungus  was  also  found  to  be  capable  of  causing 
a  rotting  of  the  fruit  similar  to  that  described  in  South  Africa.  As  the 
fungus  appeared  from  description  to  be  similar  to  the  South  African 
organism,  it  was  considered  to  be  the  same  species. 

This  form  of  gumming  is  characterized  by  gum  oozing  out  through 
cracks  in  the  bark,  usually  on  branches.     In  severe  cases  on  larger 


166 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION. 


limbs,  areas  of  bark  die  and  the  wood  is  blackened.  The  discoloration 
of  the  wood  often  extends  along  the  limbs  much  beyond  any  external 
sign  on  the  bark.  When  smaller  branches  are  affected  they  are  often 
killed  back  to  some  distance  and  numerous  black  spore  cases  push  up 
through  the  bark. 

This  kind  of  gumming  was  induced  in  more  than  twenty-five  dif- 
ferent inoculations,  covering  a  period  of  over  one  year.     That  cutting 


Fig.  8.  Diplodia  gumming 
on  young  orange  tree  after 
inoculation  with  a  bit  of  fun- 
gus. (Photo  by  O.  F.  Burger 
and  author,  Florida  Experi- 
ment   Station.) 

into  the  bark  in  itself  was  not  the  cause  of  the  gumming  was  proved 
beyond  question,  by  making  with  every  different  set  of  inoculations  the 
same  kind  of  cuts  without  the  insertion  of  the  fungus.  Such  cuts 
protected  from  contamination  always  healed  up  without  gumming.  The 
fungus  placed  on  the  uncut  or  uninjured  surface  of  branches  or  twigs, 


[Bulletin  262]  •    citrus  diseases.  167 

however,  did  not  infect  them  except  when  the  growth  was  very  soft 
and  tender. 

Pomelo  trees  in  Florida  appeared  to  be  most  susceptible  to  its  attack. 
The  fungus  probably  enters  through  injuries  or  wounds.  Cutting  out 
the  worst  diseased  limbs,  or  cutting  out  the  areas  as  they  begin  to  form 
on  one  side  of  the  larger  limbs,  or  cutting  out  smaller  branches  entirely, 
were  the  methods  used  in  controlling  this  trouble  in  Florida. 

This  fungus,  known  as  Diplodia  natalensis,  was  first  described  in 
South  Africa  in  1910  by  I.  B.  Pole  Evans  in  connection  with  a  serious 
rotting  of  lemons  and  other  citrus  fruits.  In  Florida,  the  writer  found 
this  fungus  to  be  the  cause  frequently  of  a  rotting  of  citrus  fruits, 
especially  when  slightly  injured.  It  was  also  found  to  be  the  cause  of  a 
gumming  of  peach  trees  in  Florida,  by  0.  F.  Burger.7  The  same 
fungus  was  found  to  be  capable  of  killing  the  bark  and  inducing  gum 
on  a  large  number  of  native  trees.8 

ARMILLARIA  ROOT  ROT  (Armillaria  mellea) . 

This  disease,  often  known  also  as  oak  root  fungus  and  fungus  root  rot, 
is  not  known  to  occur  on  citrus  trees  in  Florida  or  in  Cuba,  but  is 
troublesome  in  certain  sections  of  California,  especially  in  localities 
where  affected  oaks  have  been  cut  down  and  citrus  trees  planted.  The 
disease  is  due  to  a  toadstool  fungus  Armillaria  mellea. 

More  or  less  gumming  often  accompanies  this  disease.  It  is,  there- 
fore, included  under  the  gum  diseases.  The  gum  oozes  out  at  or  just 
above  the  base  of  citrus  trees,  and  might  be  confused  with  that  due  to 
foot  rot  and  gummosis.  Root  rot  is  characterized  by  a  rotting  of  the 
roots  out  to  the  ends,  and  of  the  bark  and  wood  at  the  base  of  the  tree, 
and  by  the  formation  of  black  strands  on  the  roots  and  of  fan-shaped, 
felted,  white  growths  which  crowd  into  the  live  bark.  It  is  often  ac- 
companied only  by  a  clean  mushroom  odor  and  by  the  development  of 
toadstools  of  Armillaria  mellea  near  the  base  of  the  tree  in  the  fall  and 
winter. 

The  fungus  slowly  spreads  from  the  roots  of  an  affected  tree  to  the 
roots  of  the  surrounding  healthy  trees,  often  killing  a  tree  in  each 
direction  in  from  one  to  four  years.  W.  T.  Home,9  who  has  investi- 
gated this  disease  in  connection  with  citrus  trees  in  California,  states 
that  the  treatment  of  individual  orchard  trees  after  they  are  badly 
affected  is  doubtfully  worth  while,  but  that  trees  not  girdled  by  the 
fungus  may  be  kept  alive  for  many  years  by  digging  around  the  tree, 
removing  all  diseased  roots  and  decayed  patches  of  bark  and  disinfect- 
ing the  cuts. 

In  the  handling  of  areas  or  spots  in  an  orchard  where  trees  are 
affected  or  killed  out,  it  is  necessary  not  only  to  check  the  outward 


7Mycologia  3.    151-153,    1911. 

"Florida  Agricultural  Experiment  Station,  Report  for  1912,  81-92,  1913. 

"California  State  Commission  of  Horticulture  3.,  275-282,  1914. 


168  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 

advance  of  the  fungus  so  that  new  trees  will  not  become  affected,  but 
to  treat  the  area  already  affected  before  replanting.  To  prevent  new 
trees  from  becoming  infected,  Professor  Home  suggests  digging  ditches 
so  as  to  inclose  the  affected  areas.  This  has  been  effective  in  preventing 
infection  of  new  trees  for  two  years,  even  where  the  ditches  were  filled 
up  again.  For  killing  out  the  fungus  in  the  soil  of  the  affected  area 
before  replanting,  carbon  bisulphid  1|  ounces  poured  into  holes  1^  feet 
each  way  and  1^  feet  deep  has  been  used  by  Professor  Home.  The 
use  of  carbon  bisulphid  for  killing  out  this  fungus  was  first  suggested 
by  C.  B.  Lipman. 

SUMMARY  OF  GUM   DISEASES. 

Because  of  the  liability  to  confusion  in  regard  to  these  many  forms 
of  gum  disease,  the  following  brief  summary  may  be  helpful : 

1.  Brown  rot  or  Pythiacystis  gummosis,  characterized  by  dying  of 
areas  of  bark  clear  through  to  the  wood,  accompanied  by  exudation  of 
large  quantities  of  gum,  bark  remaining  firm.  Common  and  destructive 
in  California,  especially  on  heavy  soils,  found  also  in  Florida  and  Cuba. 

2.  Mai  di  gomma  or  foot  rot,  probably  only  a  form  of  Pythiacystis 
gummosis  occurring  farther  down  at  the  base  of  trunk  and  crown  roots. 
Found  in  Florida,  California,  and  Cuba,  especially  on  old  sweet  orange 
seedlings. 

3.  Botrytis  or  grey  fungus  gummosis,  characterized  by  dying  of  a 
smaller  area  through  to  the  wood,  surrounded  by  a  larger  area  on  which 
outer  bark  only  is  killed.  Common  only  on  old  lemon  trees  in  coast 
regions  of  California.     (Not  known  in  Florida  or  Cuba.) 

4.  Psorosis  or  scaly  bark  of  California.  Found  also  in  Florida, 
mostly  on  oranges,  the  same  or  similar  form  sometimes  on  grapefruit. 
Characterized  by  patches  of  scaly,  shaggy  bark  with  gum  drops  at  their 
edges  and  with  gum-filled  outer  wood  and  the  formation  of  new  bark 
under  the  old,  making  an  ulcerated  cankerous  appearance  on  the  trunk 
or  larger  limbs. 

5.  Florida  scaly  bark,  or  nail-head  rust  (not  known  in  California 
and  Cuba)  mostly  on  sweet  orange  trees  and  fruits,  characterized  by 
small,  rusty,  glazed,  oval,  well-defined  spots  on  the  smooth,  newer  growth, 
followed  by  a  scaly  or  scabby  surface  on  the  older  bark  as  the  branches 
enlarge,  and  by  reddish  brown,  hard,  sunken  spots  on  immature  oranges 
on  the  tree,  often  in  the  form  of  rings. 

6.  Diplodia  gumming,  in  Florida  and  Cuba  (not  known  in  Cali- 
fornia) on  all  varieties  of  citrus,  characterized  by  gum  oozing  out  of 
branches  of  all  sizes,  by  a  killing  of  the  bark,  a  blackening  of  the  wood 
and  at  times  by  the  formation  of  black  spore  cases  on  the  bark. 


[Bulletin  262]  citrus  diseases.  169 

7.  Root  rot,  or  oak  root  fungus,  in  California  (not  known  in 
Florida  or  Cuba),  characterized  by  a  rotting  of  the  roots,  and  the  bark 
and  wood,  at  the  base  of  the  tree,  with  black  strands  or  rhizomorphs  on 
the  roots  and  fan-shaped  felted  white  growths  crowding  into  the  live 
bark,  accompanied  only  by  a  clean  mushroom  odor  (not  putrid  as  in 
case  of  the  foot  rot). 

DISEASES  OF  TWIGS,  LEAVES,  AND  FRUIT  EXCEPT  ROTS. 

CITRUS  CANKER  (Pseudomonas  citri) . 

This  new  disease,  which  is  attracting  serious  attention  in  Florida  and 
the  other  Gulf  States,  was  first  discovered  by  E.  W.  Berger,10  Florida 
State  Inspector  of  Nursery  Stock,  who  found  it  on  20,000  young  trees 
in  each  of  two  nurseries  in  Florida  in  the  fall  of  1912.  At  that  time  it 
was  considered  by  plant  pathologists  to  be  merely  an  unusual  mani- 
festation of  scab  and  therefore  its  seriousness  was  not  realized.  Ac- 
cording to  E.  W.  Berger,11  it  was  not  until  July,  1913,  that  he  realized 
certainly  that  he  had  discovered  a  new  disease. 

In  March,  1914,  H.  E.  Stevens,12  reported  that  he  had  found  a  species 
of  Phyllosticta  fungus  which  was  probably  the  cause  of  the  disease.  In 
May,  1914,  Wolf  and  Massey,13  of  the  Alabama  Experiment  Station, 
reported  that  they  had  proved  by  inoculation  experiments  that  a  species 
of  Phoma  was  the  cause.  Phyllosticta  and  Phoma  being  form  genera 
with  practically  the  same  characteristics,  the  fungus  obtained  in  both 
cases  was  probably  the  same.  In  April,  1915,  Clara  H.  Hasse14  pub- 
lished an  account  of  finding  a  species  of  bacterium,  Pseudomonas  citri, 
which  when  grown  in  pure  cultures  and  placed  upon  grapefruit  leaves, 
was  able  to  produce  citrus  canker.  The  published  account  of  her  work 
appears  to  leave  little  doubt  that  this  bacterium  is  the  cause  of  the 
disease.  A  letter  from  H.  E.  Stevens,  written  a  few  days  before  Miss 
Hasse 's  article  came  out,  shows  that  he  had  just  discovered  that  a  bac- 
terium and  not  a  fungus  was  the  cause  of  citrus  canker. 

The  damage  from  citrus  canker  is.  most  severe  on  grapefruit  trees,  the 
twigs,  leaves  and  fruit  of  which  are  affected.  According  to  E.  W.  Ber- 
ger, the  different  citrus  varieties  are  affected  about  in  the  following 
order :  Pomelo,  Citrus  trifoliata,  wild  lime,  Navel,  sweet  seedlings,  Sat- 
suma,  tangerine,  King  orange  and  lemon. 

Outside  of  Florida  it  has  been  found  in  Alabama,  Mississippi,  Loui- 
siana and  Texas,  and  it  is  known  to  occur  in  Japan  from  which  place  it  is 
thought  to  have  been  brought  on  nursery  stock  into  this  country.  The 
disease  has  not  been  found  in  California.  Specimens  of  the  disease, 
labeled  scab,  were  received  by  B.  F.  Floyd  at  the  Florida  Experiment 


30Florida  State   Horticultural   Society   Report,   April,    1914. 
"Florida  Agricultural  Experiment  Station  Bulletin  124,   1914. 
l2Florida  Agricultural  Experiment  Station  Bulletin  122,  March,    1914. 
l3Alabama  Agricultural  Experiment  Station  Circular  27,   1914. 
"Journal  of  Agricultural  Research  4:   97,   1915. 


170  UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION. 

Station  direct  from  Japan  in  May,  1913.  Recently  it  has  been  reported 
by  the  United  States  Department  of  Agriculture  to  be  present  in  the 
Philippine  Islands. 

The  seriousness  of  the  disease  is  apparent  from  following  statements 
in  publications  on  the  subject : 

"If  it  is  once  well  established  in  the  State  it  may  become  a  serious 
menace  to  the  grapefruit  industry"  (H.  E.  Stevens).12 

"This  disease  was  very  severe  in  certain  grapefruit  groves  during 
the  previous  season  and  threatens  to  become  the  most  serious  difficulty 
with  which  the  grower  will  have  to  contend"  (Wolf  and  Massey).13 

"This  disease  is  by  far  the  worst  which  has  ever  yet  affected  the 
citrus  industry.  The  leaves,  twigs,  and  fruit  become  covered  with  a 
cankerous  growth.  The  fruit  itself  seems  to  be  especially  susceptible 
to  the  disease,  and  drops  soon  after  becoming  diseased.  Canker  is  so 
deadly  that  when  the  tree  first  becomes  infected,  in  this  territory,  it  is 
worthless  inside  of  two  or  three  months"  (Stirling).11 

H.  E.  Stevens  in  Bulletin  124  of  the  Florida  Agricultural  Experiment 
Station,  gives  a  good  detailed  description  of  the  appearance  of  citrus 
canker  in  its  various  stages  as  follows : 

"The  distinguishing  feature  of  citrus  canker  as  observed  in  the  field  is 
the  characteristic  spotting  produced  on  the  fruit  and  foliage.  As 
usually  seen,  the  infection  appears  as  small  light-brown  spots,  from  less 
than  one-sixteenth  to  one-quarter  of  an  inch  in  diameter.  The  spots 
are  usually  round,  and  may  occur  singly,  or  several  may  run  together 
forming  an  irregular  area.  This  last  usually  occurs  on  fruits.  The 
spots  project  above  the  surrounding  healthy  tissue,  and  are  composed 
of  a  spongy  mass  of  dead  cells,  covered  by  a  thin  white  or  grayish 
membrane.  The  membrance  finally  ruptures  and  turns  outward,  form- 
ing a  lacerated  or  ragged  margin  around  the  spot. 

"On  the  leaves,  infections  first  appear  as  small,  watery  dots,  with 
raised  convex  surfaces.  These  dots  are  usually  of  a  darker  green  than 
the  surrounding  tissue.  Sometimes,  however,  the  surfaces  of  the  spots 
are  broken  as  soon  as  they  appear.  Spots  may  appear  on  either  surface 
of  the  leaf,  but  they  do  not  at  first  penetrate  through  the  leaf  tissue. 
They  gradually  increase  in  size,  change  to  a  light  brown  in  color,  and 
become  visible  on  both  sides  of  the  leaf.  In  the  older  spots  one  or  both 
surfaces  may  be  bulged  or  raised,  and  such  spots  are  commonly  sur- 
rounded by  a  narrow  yellowish  band  or  zone.  (Fig.  9.)  In  the  more 
advanced  stages,  the  surface  of  the  spots  become  white  or  grayish,  and 
finally  ruptures,  exposing  a  light  brown  spongy  central  mass.  Old  spots 
soon  become  overgrown  by  saprophytic  fungi,  and  may  appear  pink  or 
black  on  account  of  these  fungus  growths. 

"On  the  fruits  the  spots  are  very  similar  to  those  formed  on  the 
leaves.  They  do  not  penetrate  far  into  the  rind.  They  may  be  scat- 
tered over  the  surface,  or  several  may  occur  together  forming  an 
irregular  mass.  (Fig.  10.)  Gumming  is  sometimes  associated  with  the 
spots  formed  on  the  fruits.  Canker,  apparently,  does  not  cause  a  rot  of 
the  fruits  directly,  but  opens  the  way  for  other  fungi  to  enter  and  cause 

12Florida  Agricultural  Experiment  Station  Bulletin   122,   March,   1914. 
"Alabama  Agricultural   Experiment   Station   Circular   27,    1914. 
"Journal  of  Agricultural  Research  4:   97,   1915. 


[Bulletin  262] 


CITRUS   DISEASES. 


171 


Fig.   9.     Citrus   canker   on   grapefruit   twig  and   foliage.      (Stevens'    Bulletin    124, 
Florida  Experiment  Station.) 


172 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 


infected  fruits  to  rot.  The  spots  on  young  twigs  are  like  those  on  the 
leaves  and  fruit.  (Fig.  11.)  On  the  older  twigs  they  are  more  promi- 
nent, and  more  or  less  irregular  in  shape.  This  is  especially  true  of  old 
spots.  They  show  the  same  spongy  tissue  that  is  found  in  the  spots  on 
the  leaves,  but  assume  a  cankerous  appearance  and  the  surface  mem- 
brance  completely  disappears.  These  spots  or  cankers  are  formed  in  the 
outer  layers  of  the  bark  tissue,  and  do  not  penetrate  to  or  kill  the 
wood.  The  spots  once  formed  in  the  bark  are  presistent,  and  are  not 
readily  sloughed  off.  They  may  remain  for  a  long  time  and  form 
centers  from  which  infections  may  readily  spread.  This  was  confirmed 
by  observations  on  infections  produced  on  spotted  trees  in  the  green- 


Fig.   10.      Citrus  canker  on  grapefruit.      About  half  natural  size, 
letin   122,  Florida  Experiment   Station.) 


(Stevens'  Bul- 


house,  and  in  the  grove  by  artificial  infection.  Some  of  these  spots 
have  been  under  observation  for  over  a  year,  and  show  no  tendency  to 
slough  off. 

"Other  citrus  diseases  with  which  canker  may  be  confused  are  scab, 
scaly  bark,  and  possibly  Anthracnose.  It  can,  however,  readily  be 
distinguished  from  any  of  these  by  noting  the  following  points :    - 

1.  It  differs  from  scab  in  the  typically  round  spots  produced;  the 
size  of  the  spots,  and  the  fact  that  the  spots  penetrate  through  the  leaf 
tissue.  It  does  not  distort  the  leaves.  There  are.no  wart-like  projec- 
tions.    Canker  occurs  on  older  wood,  scab  does  not. 

2.  Canker  differs  from  scaly  bark  in  the  size  of  the  spots,  which  are 
much  smaller  and  more  circular  than  those   of  scaly  bark  and  the 


[Bulletin  262J 


CITRUS    DISEASES. 


173 


spongy  nature  of  the  spots — scaly  bark  spots  are  hard  and  glazed. 
Canker  is  common  on  grapefruit,  scaly  bark  is  not.  Canker  forms  spots 
on  leaves,  scaly  bark  does  not. 


Fig.  11.  Citrus  canker  on  Citrus  tri- 
foliata,  showing  the  light  colored  mem- 
brane around  the  spots.  (Stevens'  Bul- 
letin 122,  Florida  Experiment  Station.) 

3.  Canker  differs  materially  from  Anthracnose  in  the  size  of  the  spots, 
which  are  much  smaller  than  those  of  Anthracnose.  Canker  spots  are 
raised,  Anthracnose  spots  are  sunken.  Canker  has  spots  of  spongy 
character,  those  of  Anthracnose  are  hard.  Canker  occurs  on  young 
shoots  and  older  twigs,  Anthracnose  does  not." 


174  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION. 

Citrus  canker  appears  to  be  by  far  the  most  infectious  and  destructive 
disease  of  citrus  trees  yet  known.  It  is  considered  to  be  so  serious  that 
the  Florida  growers  are  spending  thousands  of  dollars  in  an  attempt  to 
eradicate  it.  The  growers'  organizations  in  the  infected  territory  have 
been  raising  large  sums  of  money,  and  the  Florida  Growers  and  Ship- 
pers League  has  been  spending  two  or  three  thousand  dollars  per  month. 
Recently  the  Federal  Government  has  appropriated  $35,000  to  assist 
the  Gulf  States,  $20,000  of  which  is  to  be  spent  in  Florida.  In  a  letter 
from  E.  W.  Berger  (February,  1915),  he  states  that  the  number  of 
inspectors  in  Dade  County,  Florida,  where  the  canker  is  most  serious, 
has  been  increased  from  about  40  to  90,  and  about  175  properties  have 
been  found  infected  in  that  county  up  to  February,  1915.  In  April, 
1915,  the  Florida  State  Legislature  passed  a  crop  pest  bill  carrying  an 
appropriation  of  $125,000  per  year  for  two  years  for  the  eradication 
of  this  disease. 

The  method  now  being  used  against  the  disease  is  complete  destruction 
of  the  infected  trees  by  burning.  When  the  control  work  first  began, 
according  to  a  report  by  Frank  Sterling  (Bulletin  124,  Florida  Experi- 
ment Station),  the  infected  trees  in  groves,  and  nursery  stock  were  first 
cut  back  and  defoliated,  and  the  trunks  painted  with  Bordeaux  or 
carbolineum.  Two  hundred  thousand  nursery  trees  and  over  five  hun- 
dred acres  of  grove  trees  were  treated  in  this  way  from  May  to  July, 
1914.  When  the  trees  put  out  again,  however,  the  new  growth  was 
infected  as  severely  as  before.  This  proved  to  be  a  complete  failure, 
due,  as  supposed,  to  infection  from  the  dead  leaves,  etc.,  from  the  sur- 
face of  the  ground.  The  method  now  employed  is  to  destroy  by  fire 
every  tree  showing  the  least  infection.  This  is  done  by  using  a  mixture 
of  kerosene  and  crude  oil  and  applying  a  torch  resembling  a  plumber's 
blow  torch  magnified  one  hundred  times.  The  tree  is  burned  to  the 
ground  and  the  surface  of  the  soil  is  thoroughly  flamed  with  the  torch. 
The  data  so  far  accumulated  since  this  method  has  been  employed,  is 
reported  to  show  that  many  groves  having  only  a  part  of  the  trees  in- 
fected have  been  successfully  freed  from  the  disease  by  burning  only 
the  infected  trees. 

MELANOSE    (Phomopsis  citri). 

This  disease  was  first  noticed  in  Florida  in  1892  and  described  by 
Swingle  and  Webber15  in  1896.  Later  it  was  studied  by  B.  F.  Floyd 
and  H.  E.  Stevens,16  of  the  Florida  Experiment  Station,  the  latter  hav- 
ing proved  it  to  be  due  to  Phomopsis  citri,  the  same  fungus  that  the 
writer  previously  proved  to  be  the  cause  of  stem-end  rot  in  Florida.17 
Strange  as  it  may  seem,  neither  Melanose  nor  stem-end  rot  have  been 


]5Division  of  Vegetable  Physiology  and  Pathology,   Bulletin  8,   1891 
]GFlorida  Agricultural  Experiment  Station,  Bulletin  111,   1912. 
17Florida  Agricultural  Experiment  Station,  Bulletin  107,  1911. 


[Bulletin  262] 


CITRUS   DISEASES. 


175 


found  in  California.  It  was  of  considerable  interest  to  the  writer  to 
find  also  that  no  Melanose  nor  stem-end  rot  could  be  found  certainly  in 
Cuba  or  on  the  Isle  of  Pines  (where  it  has  had  abundant  opportunity 
for  introduction)  and  that  as  one  passed  from  northern  to  southern 
Florida,  Melanose  appeared  to  become  less  prevalent.  At  Fort  Myers, 
Florida,  only  a  small  amount  could  be  found  and  at  Miami,  Florida, 
only  a  trace  of  it  in  a  very  mild  form.  There  seemed  to  be  some  factors, 
possibly  climatic,  causing  the  disease  to  be  abundant  in  northern  and 
central  Florida  and  causing  it  to  become  less  in  southern  Florida  and 
to  be  absent  in  Cuba. 


Fig.   12.     Melanose  spots  on  grapefruit. 


Melanose  is  a  superficial  marking  of  the  surface  of  citrus  fruits, 
leaves  and  stems.  The  most  noticeable  injury  is  that  to  the  skin  of  the 
fruits,  causing  them  to  become  rough  and  unsightly,  and  when  severe, 
stunting  their  growth.  The  markings  are  small,  raised  areas  with  a 
wax-like  appearance,  varying  from  yellow  to  brown  and  sometimes 
black.  The  individual  markings  or  specks  (varying  in  size  from  mere 
points  to  areas  one-sixteenth  of  an  inch  in  diameter)  show  under  the 
hand  lens,  lines  of  breakage  around  the  margins  or  across  the  surface, 
giving  the  appearance,  on  a  miniature  scale,  of  dry  cracked  mud,  and 


176  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION. 

when  close  together  gives  the  surface  a  flaky  appearance.  These  mark- 
ings may  be  distributed  irregularly  over  the  surface  of  the  affected 
parts  or,  as  is  often  seen,  occur  in  half  circles  or  in  lines.     (Fig.  12.) 

This  disease  occurs  on  all  varieties  of  citrus  trees  in  Florida,  but  is 
perhaps  most  noticeable  on  the  grapefruit,  because  of  the  smoothness  of 
skin  on  that  fruit.  Melanose  starts  on  leaves  and  shoots  only  when 
there  is  a  flush  of  growth.  On  the  fruit  it  may  start  at  any  time  from 
just  after  the  petals  fall  until  late  summer  or  early  fall. 

The  fungus  causing  Melanose  and  stem  end  rot,  lives  most  naturally 
in  dead  branches  and  even  in  very  small  twigs  where  in  moist  weather 
it  produces  countless  numbers  of  minute  spores.  These  spores  are 
produced  in  small  bodies  in  the  bark  that  to  the  unaided  eye  look  like 
dark  specks  or  minute  raised  pustules  on  the  surface  of  the  dead  bark. 
The  fungus  is  probably  spread  in  the  tree  chiefly  by  dew  and  rain.  The 
spores  are  washed  down  from  the  dead  twigs  to  the  fruit,  leaves  and 
new  growing  twigs.  Birds  and  insects  are  also  no  doubt  instrumental 
in  carrying  the  spores  from  tree  to  tree.  Pruning  out  dead  twigs  and 
branches  and  spraying  are  the  means  used  in  controlling  this  disease 
in  Florida. 

VERRUCOSIS    OR    SCAB  (Cladosporium  citri). 

Verrucosis,  known  as  scab,  is  a  fungous  disease  affecting  princi- 
pally the  fruit  and  leaves  of  sour  oranges  and  lemons  and  less  frequently 
Satsumas  and  grapefruit.  It  occurs  as  rough,  corky  projections  on  the 
surface  of  the  fruits,  causing  them  to  become  unsightly  and  unfit  for 
the  market.  It  attacks  the  fruit  and  leaves  when  young  and  rapidly 
growing.     (Fig.  13.)     Later  the  tissue  becomes  immune  to  attack. 

This  disease  has  never  been  found  in  California.  Before  the  strict 
quarantine  laws  were  passed,  thousands' of  sour  orange  trees  with  their 
leaves  affected  with  scab  were  brought  into  California,  but  the  new 
foliage  came  out  free  from  attack.  It  would  appear  that  this  fungus  is 
unable  to  persist  in  a  climate  like  that  of  California. 

The  effects  of  scab  are  felt  most  commercially  in  Florida  and  Cuba 
through  its  attack  on  grapefruit.  While  lemons  are  attacked  severely 
these  are  not  now  grown  commercially  in  Florida.  (Fig.  14.)  It  is 
also  severe  at  times  on  Satsumas.  It  was  of  some  interest  to  note  that 
the  scab  appeared  to  increase  in  its  injury  to  grapefruit  as  one  passed 
south  in  Florida  and  appeared  to  become  even  more  severe  and  common 
in  Cuba  and  the  Isle  of  Pines.  The  favorable  conditions  for  infection 
are  long  continued  periods  of  high  atmospheric  humidity  at  the  season 
when  the  fruit  is  forming  most  rapidly.  After  this  critical  first  growth 
period  is  over  and  the  tissue  has  hardened  even  a  little,  the  danger  from 
infection  is  over.  Scab  is  easily  controlled  in  Florida  by  the  use  of 
fungicides  and  by  care  in  cutting  down  sources  of  infection.18     W.  W. 


[Bulletin  262] 


CITRUS   DISEASES. 


177 


Fig.  13. 


Scab  on  sour  orange  leaves  in  Florida,  taken  at  Florida  Experiment 
Station.     Note  the  contortions  and  warpings  of  the  leaves. 


178 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT   STATION. 


Yothers  of  the  Bureau  of  Entomology,  states  that  the  scab  was  success- 
fully controlled  through  spraying  with  commercial  lime-sulfur  solution, 
1  part  to  30  parts  of  water. 

WITHER-TIP  OF   LIMES  (Gloeosporium  Limetticolum) . 

Wither-tip  occurs  in  its  characteristic  form  on  the  tender  foliage  of 
lime  trees.     (Fig.  15.)     This  disease  is  quite  destructive  to  this  variety 


Fig.   14.     Scab     on      lemon     fruits,      Florida.     About 
natural  size.      (Photograph,  W.  T.  Swingle.) 

of  citrus  trees  in  southern  Florida,  especially  on  the  Florida  Keys, 
where  many  lime  trees  grow.  It  is  also  common  in  Cuba.  This  mani- 
festation on  the  lime  was  found  by  Roy  E.  Clausen  to  be  due  to  an 
undescribed  species  of  Gloeosporium,  which  he  named  and  described  as 
Gloeosporium  Limetticolum.19  Although  this  fungus  is  similar  in  size 
and  shape  of  spores,  and  even  in  manner  of  spore  formation  to  Col- 
letotrichum  glceosporioides,  which  was  reported  by  P.  H.  Rolfs  to  be  the 
cause  of  this  and  also  of  other  manifestations  of  wither-tip  on  other 


18Florida  Agricultural  Experiment  Station,  Bulletin  109,  1912. 
19Phytopathology  2:  217-234,  1912. 


[Bulletin  262] 


CITRUS   DISEASES. 


179 


Fig.  15.  Wither-tip  of  lime.  1  to  7  showing  leaves  and  tips  of  new  growth  infected 
with  cultures  of  Glceosporium  Limetticolum  Clausen.  8,  9  Gloeosporium  Limetticolum 
on  agar,  dark  and  white  background.  10,  11  Calletotrichum  c/loeosporioides  on  agar, 
dark  and  white  background.      (Clausen  in  Phytopathology  II,  1912.) 


180 


UNIVERSITY    OP    CALIFORNIA EXPERIMENT    STATION. 


varieties  of  citrus  fruits,  R.  E.  Clausen  found  it  to  be  distinct.     The 
wither-tip  of  limes  has  not  been  found  in  California. 

The  method  recommended  by  P.  H.  Rolfs  for  the  control  of  this  disease 
is  pruning  out  diseased  twigs  before  the  blooming  period  and  then 
spraying  with  Bordeaux  mixture  (Bureau  of  Plant  Industry,  Bulletin 
52,  1904). 

WITHER-TIP  OF  CITRUS  TREES  OTHER  THAN  LIMES  (Collet otrichum  glac- 
osporioides) . 

A  large  amount  of  wither-tip  of  oranges,  grapefruit  and  other  citrus 
trees  (not  limes)  in  which  the  newest  growth  is  not  withered,  but  in 
which  there  is  a  slow  dying  of  twigs  and  branches,  occurs  throughout 
Florida  and  also  in  Cuba  (Fig.  16).     This  is  a  rather  serious  trouble  in 


Fig.  16. 


Pruning    for    a    severe    case    of    wither-tip    on 
orange  trees  in  Florida. 


[Bulletin  262]  citrus  diseases.  181 

many  sections  in  Florida  and,  as  worked  out  by  P.  H.  Rolfs,20  is  best 
controlled  there  by  thorough  pruning  out  of  all  dead  and  diseased  limbs 
that  show  any  signs  of  the  disease.  It  is  the  writer's  opinion,  based  on 
his  observations  in  both  Florida  and  California,  that  some  of  the  severest 
injury  in  connection  with  wither-tip  of  oranges  and  grapefruit  in 
Florida  may  be  due  to  the  combined  effect  of  Colletotrichum  glceospori- 
oides  and  other  fungi,  such  as  either  Phomopsis  citri  or  Diplodia  natal- 
ensis,  or  both.  The  last  two  fungi  have  not  been  found  in  California 
and  this  may  account  for  the  disease  not  assuming  such  severe  forms 
here.  Both  of  these  fungi  have  been  shown  to  be  capable  of  injuring 
citrus  trees  when  placed  in  cuts  in  the  branches  and  they  are  found 
abundantly  on  branches  in  Florida.  In  experiments  by  the  writer  con- 
ducted at  the  Florida  Agricultural  Experiment  Station  in  1911,  it  was 
found  that  these  fungi  in  connection  with  Colletotrichum  gloeosporioides 
were  apparently  capable  of  doing  more  damage  than  when  acting  alone. 
The  particular  strain  of  Colletotrichum  glceosporioides  used  at  that 
time  was  not  able  alone  to  do  any  appreciable  killing  of  tissue  when 
inserted  into  cuts,  but  when  combined  with  either  of  the  other  two 
much  injury  resulted.21 

Colletotrichum  gloeosporioides  is  widely  distributed  and  abundant 
in  California  as  well  as  in  Florida.  In  California  it  is  found  associated 
especially  with  dead  twigs  and  leaves  of  trees  that  have  been  weakened 
or  subjected  to  unfavorable  soil,  cultural,  or  weather  conditions. 

In  Florida  under  the  moist  weather  conditions  prevailing  in  the  sum- 
mer much  staining  of  the  fruit  is  produced  by  the  germination  of  the 
spores  of  this  fungus,  as  they  wash  down  from  dead  twigs  above.  In 
California  during  a  season  of  an  abnormal  amount  of  moisture  or  in 
localities  near  the  coast,  a  considerable  amount  of  staining  may  occur 
from  this  fungus,  but  in  drier  seasons  good  specimens  of  the  stain 
from  this  source  are  difficult  to  find.  All  varieties  of  citrus  may  be 
stained  by  this  fungus,  but  the  grapefruit  is  most  seriously  marked 
because  of  its  smooth,  lighter  colored  surface.  In  Florida  the  tear 
staining  or  tear  streaking  due  to  the  wither-tip  fungus  is  often  supple- 
mented by  Melanose  markings  and  russeting  due  to  rust  mite,  so  that 
the  fruit  is  very  unsightly  unless  it  has  been  kept  bright  by  frequent 
sprayings.  The  worst  stains  to  fruit  in  neglected  groves  in  California 
are  usually  slight  in  comparison  to  the  badly  stained  fruit  of  similarly 
neglected  groves  in  some  sections  of  Florida. 

The  same  fungus  is  also  associated  both  in  Florida  and  California 
with  a  spotting  of  oranges  and  grapefruit,  known  as  Anthracnose 
spotting  which  will  be  described  later  under  the  subject  " fruit  rots/' 
On  very  mature  fruit  it  is  also  connected  with  serious  rotting  of  fruit  in 


20Bureau  of  Plant  Industry,  Bulletin  52,  1904. 

2,Florida  Agricultural  Experiment  Station,  Report  for  1912,  p.  65. 
3—19334 


182 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 


storage  or  on  the  way  to  market.  Wither-tip  staining  of  the  fruit  is 
prevented  by  spraying  with  Bordeaux  mixture  or  ammoniacal  solution 
of  copper  carbonate. 

EXANTHEMA  OR   DIE-BACK. 

This  disease  is  found  in  both  Florida  and  California,  though  it  is  not 
so  common  in  the  latter  state.  It  was  not  seen  at  all  in  the  island  of 
Cuba  and  only  a  trace  of  it  was  found  in  the  Isle  of  Pines.  The  in- 
vestigations upon  this  disease  were  begun  by  Swingle  and  Webber  in 
Florida  and  were  later  carried  on  by  B.  F.  Floyd,  of  the  Florida 
Experiment  Station.     Die-back  is  thought  to  be  a  malnutrition  disease. 


Fig.   17.     Exanthema  or  die-back  markings  and  splitting  on 
young  green  oranges.     (About  natural  size.) 


[Bulletin  262] 


CITRUS   DISEASES. 


183 


Its  symptoms  are  various :  gum  pockets,  dark  excrescences  and  multiple 
buds  on  the  branches,  dying  back  of  branches,  and  dark  irregular  mark- 
ings on  the  skin  of  the  fruit,  with  formation  of  gum  at  the  center  of  the 
fruit  in  the  angles  of  the  divisions  (Figs.  17  and  18). 

Some  of  the  principal  causal  conditions  for  Exanthema  in  Florida 
are,  overfeeding  with  organic  nitrogenous  fertilizers,  such  as  stable 
manure,  dried  blood  and  cotton  seed  meal;  hardpan,  clay  or  marl  too 
near  the  surface ;  and  lack  of  drainage.  When  these  causal  conditions 
can  be  corrected,  the  trees  gradually  grow  out  of  the  trouble.  When 
the  disease  is  caused  by  overfeeding  with  organic  nitrogen,  the  mineral 
form  is  substituted.     It  has  been  found  best  in  Florida  to  stop  as  much 


Fig.  18.  Cross-section  of  a  green  orange  from  a  tree 
affected  with  exanthema.  The  darker  areas  between  the 
septa  next  to  the  core  indicate  the  presence  of  clear  gum 
that  has  taken  the  place  of  the  normal  tissue.     X2. 

as  possible  all  cultivation  of  the  soil  till  the  trees  have  recovered. 
Dynamiting  through  the  clay  or  hardpan  and  improving  the  drainage 
conditions  when  necessary  has  given  good  results  in  the  treatment  of 
this  disease  in  Florida.  Spraying  with  Bordeaux  mixture  both  in 
Florida  and  California  has  sometimes  been  found  beneficial  against  this 
disease,  perhaps  because  of  the  stimulating  effect  of  the  mixture. 

It  is  of  interest  to  note  that  in  California  or  in  Cuba  on  the  heavier 
soils  the  use  of  organic  nitrogen  does  not  seem,  as  a  rule,  to  bring  on 
Exanthema.  In  California,  however,  small  local  areas  of  Exanthema  in 
orchards  have  sometimes  been  found  to  correspond  with  locations  of 
former  sheep  corrals  or  cattle  barns.     In  some  places  in  California  a 


184 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 


light  sandy  soil  underlaid  with  gravelly  subsoil  is  subject  to  Exanthema. 
On  such  soils,  if  a  system  could  be  worked  out  of  mulching  with  straw 
or  vegetation  of  some  kind  and  of  irrigating  without  the  necessity  of 
such  frequent  cultivation,  it  would  probably  be  beneficial.  With  only 
the  general  methods  now  in  use,  Exanthema,  even  on  light  sandy  soils 
in  California,  appears  to  be  much  less  common  than  in  Florida  on  the 
same  type  of  soils. 

MOTTLED    LEAF. 

This  condition  of  citrus  foliage,  rather  common  in  some  sections  of 
California,  is  known  usually  as  "frenching"  in  Florida.  The  leaves 
show  yellow  areas  on  each  side  of  the  midrib  between  the  main  lateral 
veins.      (Fig.    19.)     A   nematode    worm    (Tylenchulus   semipenetrans 


Fig.   19.     Mottled    leaf    of    orange.       (Smith    Bui. 
218,    Cal.    Exp.    Sta.) 


Cobb)  was  discovered  by  J.  R.  Hodges,  and  studied  by  E.  E.  Thomas,22 
of  the  University  of  California,  in  connection  with  the  roots  of  trees 
affected  with  this  trouble.  This  nematode  worm  has  been  found  associ- 
ated with  most  cases  of  this  trouble  in  California,  but  in  only  one  or  two 
localities  in  Florida.  To  what  extent  it  is  the  cause  has  not  as  yet  been 
determined.     In  Florida,  mottled  leaf  or  "frenching"  may  be  seen  on 


'--California  Agricultural  Experiment  Station,   Circular 


191 


[Bulletin  262]  citrus  diseases.  185 

individual  trees  in  a  grove  or  on  individual  limbs  on  a  tree,  but  is  rarely 
general  throughout  any  one  grove.  The  stunting  of  fruit  resulting  in 
11  small  offs,"  as  is  sometimes  seen  in  the  worst  stages  of  mottled  leaf  in 
California,  was  not  seen  by  the  author  in  Florida  or  Cuba.  Chlorosis 
in  which  the  leaves  turn  yellow  all  over  (not  mottled)  was,  however, 
common  in  Florida  and  Cuba. 

CHLOROSIS. 

Strictly  speaking,  the  word  Chlorosis  would  also  include  mottled  leaf, 
but  it  will  be  used  here  for  a  yellowing  of  leaves  rather  uniformly  over 
the  surface  (without  mottling).  This  condition  may  arise  apparently 
from  a  lack  of  nitrogen  in  the  soil  or  other  causes.  A  great  deal  of 
Chlorosis  is  seen  in  southern  Florida  in  certain  local  areas  where  the 
soil  is  underlaid  with  marl  or  cochina  rock.  The  trees  grow  well  for  a 
time  till  their  roots  strike  the  marl,  when  their  leaves  become  yellow, 
though  not  necessarily  mottled.  This  condition  appears  to  be  corrected 
on  the  cochina  rock  soils,  by  supplying  large  quantities  of  organic  mat- 
ter for  the  purpose  of  keeping  up  the  humus.  In  California  also  in 
certain  local  areas  where  there  is  marl  near  the  surface,  Chlorosis  of 
this  type  is  seen.  There  are,  however,  many  other  cases  of  Chlorosis 
that  cannot  be  accounted  for  in  this  way. 

CITRUS    BLIGHT. 

Blight,  although  not  so  common  as  some  other  diseases,  is  the  most 
dreaded  citrus  disease  in  Florida  because  nothing  is  definitely  known 
as  to  its  cause,  and  because  it  is  likely  to  attack  suddenly  the  largest 
and  most  productive  trees  in  a  grove.  It  has  been  reported  from  Cuba, 
lbut  is  not  known  to  occur  in  California.  A  large  amount  of  scientific 
study  and  investigation  has  been  done  upon  this  disease  without  so  far 
finding  anything  that  could  satisfactorily  explain  the  cause  of  the 
trouble.  Trees  growing  on  light  hammock  soils  appear  to  be  most  sus- 
ceptible, although  it  may  occur  also  on  other  soils.  The  disease  was 
thoroughly  described  and  illustrated  by  Swingle  and  Webber  in  Bulle- 
tin 8  of  the  Division  of  Vegetable  Physiology  and  Pathology,  in  1896. 

The  first  symptom  of  blight  is  usually  a  wilting  of  the  foliage  as  if 
the  tree  was  suffering  from  drought  (Fig.  20).  Usually  this  occurs  in 
early  spring  and  at  first  appears  most  pronounced  on  dry  hot  days,  but 
later  this  wilting  continues  through  damp,  wet  weather.  Most  fre- 
quently a  single  limb  near  the  top  will  show  signs  of  wilting  after  which 
this  condition  will  spread  to  the  other  branches.  As  the  disease  pro- 
gresses, the  leaves  often  drop  off,  or  in  some  cases  the  wilting  may  occur 
so  suddenly  that  the  dry  leaves  will  remain  hanging  to  the  twigs.  After 
the  top  has  been  injured,  numerous  water  sprouts  put  out  from  the 
trunk  and  larger  limbs.  (Fig.  21.)  These  appear  healthy  at  first  and 
seem  to  give  promise  of  a  new  healthy  tree  only  to  sicken  and  die  later. 


186 


UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 


In  most  cases  of  blight,  the  roots  appear  to  be  healthy  and  a  great  deal 
of  time  has  been  wasted  in  trying  to  grow  new  tops  on  blighted  trees. 

Attempts  have  been  made  to  move  the  blighted  trees  to  different  soil 
after  cutting  back  the  top.  For  two  or  three  years  such  trees  grow  out 
and  give  promise  of  recovery,  but  later  the  top  sickens  and  dies  back  as 
before.  In  some  respects  the  disease  acts  somewhat  like  peach  yellows, 
the  cause  of  which  is  not  as  yet  known.     As  with  peach  yellows,  the 


Fig.   20. 


Branches  of  adjoining  orange  trees,     (a)    affected  with  blight;  (&)  healthy. 
(Photograph  from  Swingle  and  Webber.) 


only  remedy  is  the  total  destruction  of  the  affected  trees,  so  with  citrus 
blight  in  Florida;  the  only  remedy  so  far  used  has  been  to  dig  up  and 
destroy  a  tree  as  soon  as  it  shows  clearly  the  symptoms  of  blight. 


CITRUS  FRUIT  ROTS. 

In  Florida  the  principal  rots  and  the  fungi  that  take  part  in  each 
case  are:  (1)  blue  mold  (Penicillium  italicum),  (2)  green  mold  (Peni- 
cillium  digit at urn) ,  (3)  stem-end  rot  (Phomopsis  citri),  (4)  Anthrac- 
nose  (Collet otriclium  gloeosporioides) ,  (5)  diplodia  rot,  (Diplodia  natal- 
ensis),  (6)  black  rot  (Alt crnaria  citri) . 


[Bulletin  262] 


CITRUS   DISEASES. 


187 


Of  these  six  species  of  decay,  only  four  occur  in  California,  namely, 
blue  mold,  green  mold,  Anthracnose  and  black  rot.  In  addition  to 
these,  there  are  three  kinds  of  decay,  principally  of  lemons,  in  Cali- 
fornia which  have  never  been  known  to  cause  injury  to  citrus  fruits  in 
Florida;  brown  rot  (Pythiacystis  citrophthora) ,  cottony  rot  (Sclerotinia 
libertiana),  and  grey  fungus  rot   (Botrytis  cinerea).     The  brown  rot 


Fig.   21.     Orange  tree  that  has  been  affected  with  blight  for 
some  time.     Suckers  from  base  of  tree  growing  up  through  it. 

fungus  has  been  found,  as  was  before  stated,  in  Florida  in  connection 
with  gum  disease,  but  it  has  been  reported  only  once  on  fruit  and  that 
doubtfully,  on  a  few  orange  fruits  at  Miami,  Florida. 

BLUE  AND  GREEN   MOLDS  (Penicillium  italicum  and  P.  digitatum) . 

These  molds  caused  by  two  closely  related  fungi  are  the  most  common 
of  all  rots  in  both  Florida  and  California.  These  fungi  are  apparently 
unable  to  enter  except  through  imperfections  or  injuries  to  the  skin  of 
the  fruit.  It  has  been  found  by  many  experiments  that  a  large  part  of 
this  decay  may  be  avoided  by  careful  handling  at  all  stages  of  picking, 


188  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION. 

packing  and  shipping  of  the  fruit.23  The  Florida  fruit  having,  as  a 
rule,  a  thinner  skin  and  being,  as  a  rule,  less  firm  than  the  California 
orange,  it  is  apparently  somewhat  more  subject  to  injury  and  decay 
of  this  nature  than  the  latter. 

STEM -END  ROT  (Phomopsis  citri). 

Next  to  the  blue  and  green  molds,  this  decay  is  the  most  common  and 
troublesome  rot  in  Florida  during  certain  seasons,  when  conditions  are 
right  for  its  development.  Although  fruits  sometimes  begin  to  decay  at 
the  stem  end  in  California  by  other  causes,  the  form  caused  by  this 
fungus  is  not  known  to  occur  in  California,  nor  in  Cuba.  The  writer 
began  an  investigation  of  this  disease  in  Florida  in  1909  and  found 
it  to  be  due  to  an  apparently  new  species  of  fungus  which  he  described 
as  Phomopsis  citri.24  Later  H.  E.  Stevens  found  that  melanose  was  due 
to  the  same  fungus.  While  melanose  is  a  marking  of  the  surface  of  the 
fruit  produced  by  the  fungus  when  the  fruit  is  in  an  immature,  rapidly 
growing  condition,  stem-end  rot  is  a  decay  mostly  of  the  interior  of  the 
fruit  just  before  or  after  it  is  mature.  The  injury  to  the  fruit  is  two- 
fold: first,  it  causes  the  fruit  to  drop  and  rot  just  before  or  after  its 
maturity  on  the  tree ;  and  secondly,  it  causes  a  softening  and  rotting  of 
the  fruit  in  transit,  or  soon  after  arrival  at  the  market.  This  disease 
may  occur  on  all  varieties  of  citrus  fruits. 

In  the  grove  in  Florida,  stem-end  rot  first  appears  in  August  or 
September  on  immature  oranges  or  grapefruit  as  a  dark  brown,  reddish- 
brown,  or  black  discoloration  about  the  base  of  the  fruit.  More  often 
than  not,  the  fruits  drop  off  before  any  discoloration  begins.  When  the 
disease  attacks  the  mature  fruit,  a  circular  patch  at  the  base  becomes 
soft  without  discoloration.  As  the  softened  area  enlarges,  covering  one- 
third  to  one-half  of  the  surface  of  the  fruit,  the  yellow  or  orange  color 
of  the  rind  changes  through  dull  brown  to  dark  coffee  color.  On  open- 
ing the  fruit  the  decay  will  be  found  to  have  proceeded  most  rapidly 
along  the  fibrous  core  in  the  center  and  along  the  white  inner  part  of 
the  peel.  This  rotting  may  cause  much  destruction  of  packed  fruit 
while  in  transit  and  after  arrival  on  the  market.  Shipping  tests  of 
carefully  picked  and  packed  fruit  showed  an  amount  of  stem-end  rot 
from  zero  up  to  10  per  cent  immediately  on  arrival  at  Washington ;  0.6 
to  30  per  cent  one  week  after  arrival,  8  to  52  per  cent  two  weeks  after, 
and  18  to  68  per  cent  three  weeks  after.25 

The  fungus  causing  stem-end  rot  is  capable  of  attacking  the  fruit  at 
the  stem  even  when  there  is  no  injury,  although  certain  conditions,  such 
as  the  presence  of  scale  insects,  humidity,  heat  and  lack  of  vigor  of  the 
tree,  appear  to  be  predisposing  factors  in  bringing  on  this  decay.     Since 


23Powell,  J.  H.,  Bureau  of  Plant  Industry,  Bulletin  123. 

2<Phytopathology  2,   109-113,  1912. 

25Florida  Agricultural  Experiment  Station,  Bulletin  107,  1911. 


[Bulletin  262]  citrus  diseases.  189 

the  fungus  produces  an  abundance  of  spores  on  the  dead  branches  and 
limbs,  the  elimination  of  these  as  far  as  possible  is  being  practiced. in 
Florida  for  its  control. 

ANTHRACNOSE    (Colletotrichum  glceosporioides) . 

This  form  of  decay  also  known  as  wither-tip  spotting  and  decay,  is 
especially  destructive  to  grapefruit  during  some  seasons  in  Florida.26 
It  also  occurs  in  Cuba  and  in  California  during  some  seasons.  It  mani- 
fests itself  in  circular,  sunken  spots  on  the  rind  of  the  fruit  which  are 
at  first  firm.  On  very  mature  or  weak  fruit  the  fungus  either  of  itself 
or  by  opening  up  the  way  for  other  organisms  is  able  to  produce  a  soft 
rot,  especially  in  packed  fruits  on  the  way  to  market.  The  same  fungus 
is  associated  with  it  as  with  wither-tip  and  tear  staining,  which  have 
previously  been  mentioned.27 

BLACK    ROT  OF  ORANGES  (Alternaria  citri). 

This  is  a  disease  of  oranges  that  begins  at  the  " blossom"  or  stylar 
end.  It  is  not  at  all  common  in  Florida,  but  is  prevalent  in  certain 
parts  of  California  and  Arizona,  where  Navel  oranges  are  grown.28  It 
is  due  to  a  fungus  that  enters  the  navel  end  of  the  fruit  while  it  is 
young.  It  will  sometimes  cause  young  green  fruits  to  exude  drops  of 
gum  and  turn  yellowish  at  the  navel  end  and  drop  off.  The  rot  is  most 
often  noticed  just  before  the  normal  crop  colors.  The  infected  fruits  at 
this  time  color  up  sooner  than  the  sound  fruits  and  turn  a  deep  orange 
color.  To  outside  appearance  they  look  good,  but  when  cut  open  are 
found  to  have  a  black  decayed  center.  The  disease  has  been  observed 
in  Florida  on  other  varieties  than  the  Navel,  where  it  seemed  to  have 
begun  in  a  slight  defect  at  the  blossom  end.  The  disease  does  not 
usually  attract  enough  attention  to  call  for  remedial  measures.  Only  a 
small  per  cent  of  the  fruit  is  attacked.29  In  one  small  grove,  however, 
in  northern  California,  in  1914,  fifty  per  cent  of  the  fruit  were  reported 
to  be  affected  with  black  rot. 

DIPLOPIA  ROT   (Diplodia  natalensis). 

This  form  of  decay  usually  starts  at  the  stem  end,  as  does  the  stem-end 
rot,  and  is  similar  in  appearance  at  first  to  the  stem-end  rot,  due  to 
Phomopsis  citri.  It  differs  by  the  discoloration  becoming  darker  as 
the  decay  proceeds  and  usually  showing  dark,  wide  bands  corresponding 
to  the  septa  or  divisions  between  the  segments.  It  is  of  minor  im- 
portance in  Florida  and  has  not  been  found  in  California.  It  is  most 
common  in  Cuba.  The  writer  once  received  specimens  of  this  decay 
from  Porto  Rico.  It  was  first  described  as  causing  a  serious  decay  of 
lemons  in  South  Africa.30 


26Hume,  H.  H.,  Florida  Agricultural  Experiment  Station,  Bulletin  77,  1904. 
27Rolfs,  P.  H.,  Bureau  of  Plant  Industry,  Bulletin  52,  1904. 

28 Arizona  Agricultural  Experiment  Station,  Bulletin  58,  and  California  Agricultural 
Experiment  Station,  Bulletin  218. 

29Amundson,  California  State  Commission  of  Horticulture,  2,  527-534,   1913. 
4—19334 


190  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 

BROWN    ROT    (Pythiacystis  citrophthora) . 

This  form  of  rot  occurs  in  California  and  most  commonly  on  lemons. 
It  is  not  of  any  economic  importance  in  Florida  and  Cuba,  but  has  been 
reported  doubtfully  as  occurring  on  fruits  in  one  locality  in  each  place. 
It  is  a  decay  of  brown  color  in  which  the  tissue  of  the  fruit  remains 
more  firm  than  in  most  other  rots.  When  in  the  open  there  is  no  visible 
growth  of  fungus  on  the  surface.  The  fungus  develops  in  the  soil  and 
the  low  hanging  fruits  on  the  trees  are  affected  by  the  splashing  of 
muddy  water  during  rains  from  the  surface  of  the  soil.  It  is  prevented 
in  the  orchard  by  spraying  the  ground  and  lower  branches  with  Bor- 
deaux mixture  or  by  covering  it  with  straw  in  the  rainy  season,  and  in 
the  packing-house  by  using  copper  sulphate  at  the  rate  of  1  pound  to 
800  gallons  in  the  washing  tanks.31 

COTTONY  ROT  (Sclerotinia  libertiana). 

Another  rot  of  lemons  in  California  that  does  not  cause  any  trouble 
as  far  as  known  in  Florida  or  Cuba,  is  the  cottony  rot.  The  fruits  are 
softened  and  covered  with  a  white  mold  growth  which  later  forms  large 
irregular  black  seed-like  bodies,  the  sclerotia.  (Fig.  22.)  The  fungus 
also  sometimes  attacks  the  young  shoots  of  citrus  trees,  especially 
lemons,  causing  the  bark  to  soften  and  the  limbs  to  gum  and  die-back.32. 

GREY    MOLD   (Botrytis  cinerea) . 

The  grey  mold  or  Botrytis  decay  begins  as  a  dark  discoloration,  the 
fruit  then  softens  and  a  mouse  grey,  furry  growth  of  the  fungus 
develops  over  the  surface.  This  decay  is  confined  principally  to  lemon 
fruits  in  the  packing-houses  in  California.  It  is  not  known  to  affect 
citrus  fruits  in  Florida  or  Cuba. 

There  are  in  addition  to  these  rots  a  number  of  surface  spots  and 
markings  and  pittings  of  fruit,  a  few  of  which  will  be  spoken  of  here. 
A  more  complete  list  will  be  found  on  pages  207-210,  at  the  end  of 
the  bulletin. 

BLACK  PIT  {Pscudomonas  citriputcale) . 

This  is  a  black  sunken  spot  of  citrus  fruits  in  California  due  to  a 
bacterium.  The  cause  was  worked  out  by  C.  O.  Smith33  of  the  Southern 
California  Pathological  Laboratory.  The  organism  is  thought  to  gain 
entrance  at  slight  injuries,  such  as  thorn  punctures  and  the  like.  It  is 
only  a  minor  trouble.  As  far  as  known,  it  does  not  occur  in  Florida  or 
Cuba. 


3oEvans,  I.  B.  P.,  Transvaal  Department  of  Agricultural  Science,  Bulletin  4,  1910. 
3,California  Agricultural  Experiment  Station,  Bulletin  190,  1907. 
32Smith,   C.   O.,   California  Cultivator,   35:    196-197,   September   1,    1910. 


[Bulletin  262] 


CITRUS   DISEASES. 


191 


BROWN   SPOT. 

A  spot  disease  occurring  principally  on  Navel  orange  fruits  in  Cali- 
fornia is  known  by  this  name.  The  spots  vary  in  size  from  a  mere  point 
to  one-fourth  inch  in  diameter ;  they  are  dark  brown  and  sunken,  begin 
to  develop  five  to  ten  days  after  the  fruit  is  picked,  and  are  usually  not 
visible  on  the  tree.  J.  E.  Coit34  attributes  the  spotting  to  the  premature 
death  of  certain  cells  near  to  surface  of  the  rind  due  to  oxidizing 
enzymes.  The  primary  cause  or  agent  that  brings  about  this  result  is 
as  yet  unknown. 


Fig.   22.     Cottony    rot    on    lemon    showing   mycelium    and 
sclerotia  of  Sclerotinia  libertiana.      (Smith,  Bui.  218.) 


PETECA. 

This  occurs  on  lemon  fruits  as  a  deep  pitting,  due  to  the  sinking  of 
the  surface  of  the  rind  after  the  fruit  has  been  some  time  in  the  curing 
house.  Although  the  surface  cells  at  the  bottom  of  the  pits  are  at  first 
normal,  the  tissue  underneath  these  in  the  inner  part  of  the  peel  are  dry 
and  shrunken.     This  trouble  appears  to  occur  only  during  the  fall  and 


33Phytopathology  3,    277-281,    1914. 

34Proceedings  of  Society  for  Horticultural  Science  for  1910. 


192 


UNIVERSITY    OP    CALIFORNIA — EXPERIMENT    STATION. 


winter  months  in  California  and  disappears  on  the  lemon  fruits  of  other 
seasons.  It  is  not  known  to  occur  in  Florida  or  Cuba,  but  is  said  by 
Ralph  E.  Smith,  to  occur  in  southern  Europe.  No  cause  or  remedy  for 
this  disease  is  yet  known.     (Fig.  23.) 

GREEN   SPOT. 

A  marking  of  the  surface  of  lemons  known  as  "green  spot"  is  trouble- 
some during  some  years  in  California.  It  is  not  known  to  occur  in 
Florida  or  Cuba.  It  usually  occurs  in  the  fall  and  winter  on  the  fruit 
that  has  come  in  green  or  light  green  from  the  orchard.  The  spots 
remain  green  in  color  after  the  remainder  of  the  surface  of  the  rind 
turns  yellow  in  curing.  In  the  spots  the  oil  cells  stand  out  promi- 
nently, the  surface  between  them  having  been  slightly  depressed.     It 


Fig.  23.     Peteca  of  lemons.      (Smith,  Bui.  218.) 

has  been  found  recently  that  one  of  the  causes  for  these  spots  is  the 
effect  of  lemon  oil  liberated  from  some  of  the  cells  of  the  rind  in  pick- 
ing and  handling.  J.  D.  Culbertson,  of  the  Limoneira  Company,  first 
showed  by  experiments  on  sound  lemons  that  the  oil  pressed  out  of 
the  rind  of  one  fruit  had  a  remarkable  effect  on  the  rind  of  another 
fruit  uninjured.  The  writer  caused  the  development  of  the  typical 
"green  spot"  in  moist  jars,  by  pressing  out  on  the  uninjured  surfaces 
of  green  fruits  small  quantities  of  lemon  oil.  The  spots  were  also 
produced  by  a  pressure  of  the  skin  just  sufficient  to  liberate  the  oil. 
Fruits  not  so  treated  kept  under  similar  conditions  developed  no  green 
spots.  The  depression  of  the  surface  of  the  rind  between  the  oil  cells 
took  place  in  a  few  hours,  but  the  typical  green  spot  was  only  evident 
in  four  to  six  weeks,  after  the  remainder  of  the  rind  had  turned  yellow. 


[Bulletin  262] 


CITRUS   DISEASES. 


193 


THE  PRINCIPAL  INSECT  PESTS  OF  FLORIDA  AND 
CALIFORNIA  COMPARED. 

WHITE   FLIES. 

Only  the  more  important  insect  diseases  of  citrus  trees  will  be  men- 
tioned. In  Florida  the  most  serious  pest  in  most  sections,  is  the  citrus 
white  fly.  In  importance,  this  pest  holds  in  Florida  the  place  that 
the  black  scale  holds  in  southern  California.  There  are  two  species 
to  which  the  name  citrus  white  fly  is  usually  given,  Aleyrodes  citri 
and  Aleyrodes  nubifera.  There  is  a  third  species  usually  known  as  the 
Cuban  or  woolly  white  fly,  Aleyrodes  howardii,  that  is  now  spreading 
over  all  the  southern  part  of  Florida  in  eight  counties. 


Fig.  24.  Citrus  white  fly  parasitized  by  fungi.  (A)  Brown  fungus  (fflgerita  web- 
beri.  (B)  Red  fungus  (Aschersonia  aleyrodis).  Each  pustule  indicates  the  position 
of -a  white  fly  larva  that  has  been  killed  by  the  fungus.      (H.  J.  Webber.) 


194  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 

The  citrus  white  fly  in  its  adult  stage  is  a  gnat-like  insect.  The  eggs 
are  laid  upon  the  under  side  of  the  citrus  leaves  and  hatch  into  flat, 
scale-like  larvae,  which  suck  the  sap  from  the  leaves.  The  injury  to 
the  tree  is  caused  both  by  the  sucking  of  the  sap  and  by  the  black 
sooty  mold  fungus,  Meliola  sp.,  that  grows  in  the  secretion  of  honey  dew 
from  the  white  fly  larvae.  When  there  is  a  bad  infestation  of  this 
insect,  the  surface  of  the  leaves,  twigs  and  fruit  become  coated  with  a 
layer  of  the  sooty  mold  fungus  in  a  manner  similar  to  that  following 
a  very  bad  attack  of  black  scale  (Saissetia  olece  Bern.)  or  mealy  bugs 
(Pseudococcus  citri  Risso.)  in  California.  The  white  fly  has  three 
well-defined  broods  or  cycles  in  Florida.  The  adults  of  the  first  brood 
hatch  and  are  on  the  wing  during  March  and  April,  the  second  during 
June  and  July,  and  the  third  during  August,  September  and  October. 

It  is  interesting  to  note  the  manner  in  which  the  white  flies  are 
attacked  by  fungi  in  Florida.  There  have  been  found  six  different 
species  of  fungi  that  are  parasites  of  the  citrus  white  fly  in  Florida, 
namely,  Aschersonia  aleyrodis  (Fig.  24A)  Aschersonia  flavo-citrina, 
JEgerita  webberi  (Fig.  24B),  Microcera  (Fusarium)  sp.,  Verticil- 
Hum  heterocladum,  and  Sphcerostilbe  coccophila.  The  first  four  of 
these  are  quite  efficient  in  helping  to  keep  the  insect  in  check  under 
favorable  conditions  of  moisture  and  temperature.  The  first  three 
attack  the  larvae  or  scale-like  stage  of  the  white  fly.  Microcera  sp. 
attacks  larvae  and  eggs.  "When  the  fungi  become  thoroughly  estab- 
lished in  a  moist  locality  the  grower,  without  any  other  aid,  usually 
counts,  during  average  conditions,  on  at  least  one  clean  crop  in  three 
years.  This  is  thought  to  be  due  to  the  fact  that  the  fungi  have 
become  so  abundant  one  year  in  three  that  the  insects  are  nearly  all 
killed.  The  following  year  the  trees  are  practically  clean  because 
there  are  very  few  insects  left.  The  fungi,  however,  having  used  up 
the  food  supply  the  year  before,  have  become  scarce.  The  few  insects 
which  remain  increase  rapidly  and  the  next  year  become  numerous 
enough  to  smut  the  fruit  somewhat,  and  the  second  year  so  numerous 
as  to  smut  the  fruit  badly,  but  by  this  time  the  fungus  parasites  have 
again  increased  to  such  an  extent  as  to  nearly  exterminate  them  again. 
The  third  year  is  therefore  again  a  year  with  few  insects  and  clean 
fruit.  If  conditions  are  not  normal,  this  three-year  period  may  be 
changed  to  a  shorter  or  longer  period,  depending  largely  on  weather 
conditions.  In  some  unusually  moist  situations  the  fly  may  be  practi- 
cally controlled  by  the  fungi  for  several  years  in  succession  without 
other  aid.  When  a  dry  year  occurs  the  fungi  are  unable  to  keep  the 
white  flies  in  check.  The  fungi  do  most  of  their  work  in  the  rainy 
season  and  the  insects  multiply  most  in  the  dry  season  or  during 
periods  of  lack  of  moisture.  If  the  rainy  season,  therefore,  lasted 
all  the  year  in  Florida,  the  white  fly  would  probably  be  sufficiently 


[Bulletin  262]  citrus  diseases.  195 

controlled  by  the  fungi  without  any  other  aid.  It  would,  therefore, 
seem  that  if  the  white  fly  once  became  established  in  the  drier  climate 
of  California  where  the  parasitic  fungi  probably  would  not  thrive,  it 
would  be  a  much  more  serious  pest  than  it  now  is  in  Florida.  We 
have  one  insect,  the  black  scale  (Saissetia  olece)  occurring  in  both  Cali- 
fornia and  Florida,  whose  occurrence  in  the  two  states  indicates 
something  as  to  what  kind  of  a  pest  we  could  expect  the  white  fly  to 
become  in  the  drier  climate  of  California  where  its  fungus  parasites 
would  not  thrive.  The  black  scale  is  probably  the  most  serious  general 
insect  in  southern  California,  yet  in  Florida  and  Cuba,  it  is  not 
abundant  enough  to  be  of  any  consequence  whatever.  It  is  probably 
controlled  largely  in  those  places  by  both  fungus  and  insect  parasites. 
Even  in  moister  regions  of  California,  as  in  Santa  Barbara  County, 
the  black  scale  appears  to  be  partially  held  in  check  by  a  species  of 
Isaria  fungus,  assisting  the  Scutillaria  and  other  parasitic  insects. 

A  method  has  been  in  use  for  some  years  in  Florida  of  aiding  in  the 
distribution  of  the  fungi  by  spraying  the  trees  with  water  in  which 
spores  of  these  fungi  have  been  placed.  This  method  was  first  brought 
out  by  Dr.  E.  W.  Berger  of  the  Florida  Experiment  Station,  and  at 
the  present  time  hundreds  of  acres  are  being  treated  in  this  way. 
Spraying  with  oil  sprays  is  also  being  generally  used  in  controlling 
this  insect.  W.  W.  Yothers,  of  the  Bureau  of  Entomology,  has  de- 
veloped a  heavy  oil  spray,  an  emulsion  of  lubricating  oil  in  whale-oil 
soap,  that  is  now  being  used  against  this  pest  and  against  scale  insects 
as  well.  The  work  of  the  fungi  is  often  supplemented  by  spraying 
at  periods  when  the  fungi  are  unable  to  keep  the  insect  in  check.  As 
regards  fumigation  with  hydrocyanic  acid  gas,  Professor  J.  R.  Watson, 
Entomologist  of  the  Florida  Experiment  Station,  says:  "So  far  as  I 
know  at  the  present  time  (November,  1914),  there  is  not  a  grower  in 
Florida  who  fumigates  regularly."  This,  to  be  successful,  would  have 
to  be  done  by  communities,  because  of  the  possibility  of  the  adult  fly 
reinfecting  a  clean  grove  by  flying  over  from  a  neighboring  infested 
one.  Not  all  communities  in  Florida  are  yet  infested  with  the  citrus 
white  fly,  but  year  by  year  it  is  spreading  to  new  places.  The  wonder 
is  that  some  places  only  a  few  miles  separated  from  others  badly 
infested,  have  remained  free  for  so  many  years. 

The  white  fly  at  one  time  became  established  in  three  widely  sep- 
arated localities  in  California:  Bakersfield,  Oroville,  and  Marysville — 
but  by  severe  measures,  it  has  apparently  been  entirely  eradicated, 
except  on  a  few  trees  at  Marysville. 


196  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION. 

THE  PURPLE  SCALE  (Lepidosaphes  oeckii). 

Next  to  the  white  fly  the  purple  scale,  Lepidosaphes  beckii,  is  the 
most  important  insect  pest  in  Florida.  This  insect  is  widely  dis- 
tributed throughout  all  the  citrus  localities  of  the  state  and  at  times 
becomes  very  troublesome.  This  scale  is  attacked  by  at  least  four 
different  fungus  parasites,  Microcera  (Fusarium)  sp.,  Sphcerostilbe 
coccophila,  Ophionectria  coccicola  and  Myriangium  duricei,  and  some- 
times by  a  fifth,  V  erticillium  heterocladum.  These  fungi,  together 
with  insect  parasites  under  quite  favorable  moisture  conditions  in 
Florida,  keep  the  purple  scale  down  so  that  it  does  little  damage  to  the 
trees  and  fruit.  During  a  normal  dry  season,  generally  during  the 
winter  and  spring  in  Florida,  especially  in  higher  localities  or  in 
regions  exposed  to  drying  winds,  the  fungi  are  unable  to  keep  pace 
with  the  insects  and  they  become  very  abundant  and  injurious.  When 
groves  that  have  never  had  the  white  fly  in  them  first  become  thoroughly 
infested  with  this  insect,  the  purple  scale  at  first  also  increases  con- 
siderably. J.  R.  Watson,35  has  recently  explained  this  increase  by 
the  fact  that  the  crawlers  or  young  are  driven  by  strong  light  to 
seek  shelter  under  the  sooty  mold  or  under  the  calyx  of  the  fruit,  etc., 
and  having  once  taken  up  their  position  there,  are  protected  from  their 
enemies,  the  lady  beetles  and  perhaps  also  from  the  fungi.  The  sooty 
mold  would  naturally  act  as  does  cotton  in  preventing  the  passage  of 
wind-blown  spores  through  it  to  the  insects  underneath. 

In  California  the  same  species  known  as  purple  scale  is  found  princi- 
pally in  the  coast  regions  of  Santa  Barbara,  Los  Angeles,  Orange  and 
San  Diego  counties,  but  none  of  the  fungus  parasites  before  men- 
tioned are  present  here.  It  is  also  an  important  pest  in  these  places. 
A  number  of  attempts  by  the  author  to  introduce  the  red  fungus 
(Sphcerostilbe.  coccophila)  on  purple  scale  into  Santa  Barbara  and 
San  Diego  counties,  have  failed  completely.  It  is  likely  that  the 
climatic,  conditions  of  California  are  not  suitable  for  the  growth  and 
development  of  this  fungus. 

OTHER   SCALE    INSECTS. 

Other  insects  in  Florida  which  at  times  are  important  pests  are 
the  long  scale  (Lepidosaphes  gloverii),  Florida  red  scale  (Chrysom- 
phalus  aonidtim),  chaff  scale  (Parlatoria  pergandei) ,  cottony  cushion 
scale  (Icerya  purchasi) ,  and  mealy  bug  (Pseudo coccus  citri).  Only 
the  last  two  of  these  are  found  to  any  extent  in  California.  In  Cali- 
fornia only  a  small  amount  of  the  long  scale  is  present  in  San  Diego 
County.  The  ''red  scale"  of  California  is  a  different  species  (Chrysom- 
phalus  aurantii),  and  other  scales  of  considerable  importance  in  Cali- 
fornia in  certain  localities  are  the  yellow  scale  known  as  a  variety  (var. 


35Florida  Agricultural   Experiment   Station   Bulletin   123,    1914. 


[Bulletin  262]  citrus  diseases.  197 

citrinus)  of  the  red  scale,  greedy  scale  (Aspidiotus  rapax)  and  a  com- 
paratively new  pest,  Citricola  scale  (Coccus  citricola).  In  California, 
fumigation  with  hydrocyanic  acid  gas  or  spraying,  and  in  a  few  cases 
insect  parasites,  are  the  means  used  in  controlling  scale  insects.  In 
Florida,  fumigation  is  used  scarcely  at  all  and  the  fungus  parasites 
supplemented  by  spraying  are  employed. 

Besides  these,  a  number  of  other  insects  are  mentioned  in  comparison 
with  those  of  California  and  Cuba  on  pages  203-206  with  their  known 
fungus  and  insect  parasites. 

RED  SPIDERS  AND   MITES. 

There  are  three  species  of  mites  that  are  troublesome  and  widely 
distributed  all  over  Florida,  viz:  the  red  spider,  Tetranychus  mytilas- 
pidis,  the  six-spotted  mite,  Tetranychus  sexmaculatus,  and  the  rust 
mite,  Eriophyes  oleivorus.  The  red  spider  is  the  same  pest  that  is  so 
troublesome  in  California,  the  other  two  are  not  widely  distributed  in 
California  and  are  found  only  to  a  limited  extent  in  San  Diego  County. 
All  these  mites  are  successfully  controlled  in  Florida  as  in  California, 
by  spraying  with  lime-sulfur,  or  by  using  powdered  sulfur  in  hot,  damp 
weather.  An  interesting  observation  that  has  been  made  by  the  growers 
for  years  in  Florida,  and  which  has  also  been  noticed  by  the  author 
many  times,  is  that  the  rust  mite  seems  to  be  much  more  prevalent  on 
high  pine  lands  than  on  lower  hammock  lands,  and  also  appears  to  be 
worse  on  groves  that  have  been  clean  cultivated  than  on  groves  where 
the  weeds  or  cover  crops  are  allowed  to  grow  throughout  the  summer. 
This  same  difference  as  to  amount  of  russeted  fruit  between  mulched 
and  unmulched  trees  was  also  observed  in  Cuba.  The  fruit  on  the 
mulched  trees  were  almost  invariably  freer  from  attack  of  rust  mite  than 
the  fruit  on  other  trees  not  mulched  in  the  same  soil.  No  satisfactory 
explanation  of  this  difference  is  known  to  the  author.  It  may  possibly 
be  due  to  there  being  parasites  of  the  mites  that  keep  them  down  in 
lower  situations  and  that  are  not  so  abundant  in  cultivated  places  or  in 
clean  cultivated  lands. 


198  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 


CITRUS  DISEASES  IN  CUBA  AND  THE  ISLE  Of  PINES. 


INTRODUCTION. 

About  two  weeks  were  spent  in  Cuba  and  the  Isle  of  Pines.  As  the 
localities  where  citrus  is  grown  are  scattered  from  one  end  of  Cuba  to 
the  other,  there  was  not  time  enough  to  visit  more  than  a  few  of  the 
localities  where  citrus  fruits  were  grown.  The  localities  visited  were 
Herradura  in  the  province  of  Pinar  del  Rio,  Santiago  de  las  Vegas  in 
the  province  of  Havana,  Ceballos  in  the  Camaguey  province,  and  Santa 
Fe,  McKinley  and  Santa  Barbara  in  the  Isle  of  Pines,  which  were  said 
to  be  representative  localities. 

According  to  statistics  published  by  the  Cuba  News  of  Havana  in  1913 
there  were  nearly  20,000  acres  of  citrus  fruits  then  growing  commer- 
cially in  Cuba  and  the  Isle  of  Pines,  most  of  it  ranging  in  age  from  one 
year  to  about  ten  years.  This  does  not  include  the  older  scattered  citrus 
trees  throughout  the  islands.  In  round  numbers,  about  14,000  acres  of 
this  are  grapefruit,  5,000  acres  are  orange,  and  800  acres  are  lemons. 
Most  attention  is  being  paid  to  grapefruit  for  shipping,  oranges  being 
grown  principally  for  local  consumption,  and  little  attention  is  now 
being  paid  to  lemons. 

In  climatic  conditions,  Cuba  and  the  Isle  of  Pines  are  similar  in  many 
respects  to  southern  Florida.  The  rainy  season  occurs  in  summer  and 
the  drier  period  in  winter,  just  as  in  Florida.  Cuba  and  the  Isle  of 
Pines,  being  between  20  and  23  degrees  north  latitude,  have  one  great 
advantage  over  most  of  the  citrus  regions  of  Florida  and  California,  viz, 
the  total  freedom  from  frost  injury.  The  most  tender  varieties  of  citrus 
and  tropical  fruits  may  be  raised  without  danger  from  such  injury. 

One  very  serious  injury  aside  from  the  insect  pests  and  diseases  to 
citrus  growing  in  Cuba  during  some  seasons,  however,  is  heavy  winds, 
which  gain  a  velocity  at  times,  to  the  extent  of  becoming  hurricanes. 
These  cause  much  injury  to  groves  that  are  exposed.  The  fruit  is 
whipped  about  and  the  trees  are  severely  injured.  This  injury  has  in 
some  places  been  prevented  in  great  measure  by  the  use  of  suitable 
windbreaks  and  much  interest  is  being  taken  in  the  planting  of  wind- 
breaks at  the  present  time. 

INSECT  PESTS. 

BLUE-GREEN    BEETLES. 

Perhaps  one  of  the  most  serious  pests  in  the  islands  are  the  blue-green 
beetles,  Pachnceus  lit  us  and  P.  azurescens,  that  eat  out  small  patches  of 
the  rind  of  the  young  fruits  and  cause  them  to  become  misshapen.  The 
larvae  of  these  beetles  live  in  the  soil  and  eat  the  bark  from  roots  of 


[Bulletin  262]  citrus  diseases.  199 

the  trees.  The  beetles  usually  begin  to  appear  at  the  time  of  the  early 
rains  in  April  and  May  and  continue  in  decreasing  numbers  until 
October.  The  injury  from  the  larvae  on  the  roots  is  said  to  show  most 
in  the  dry  season  in  winter  and  early  spring.  The  trees  turn  yellow 
and  in  severe  cases  die.  No  satisfactory  practical  method  appeared  to 
have  been  used  to  any  extent  for  controlling  these  beetles  in  large  trees. 
For  small  trees,  shaking  the  beetles  off  upon  a  sheet  stretched  under  the 
tree  and  killing  them,  had  been  tried  with  fair  success. 

LEAF-CUTTING   ANT. 

Another  pest  of  constant  annoyance  is  the  leaf-cutting  ant  known  as 
the  ' '  bibi jaga, ' '  Atta  insularis.  A  colony  of  these  ants  in  one  night  may 
completely  defoliate  a  number  of  citrus  trees.  The  ants  use  the  leaves 
in  making  a  pulp  or  culture  medium  for  a  certain  species  of  fungus 
which  they  cultivate  with  great  care  for  their  food  in  underground  nests. 
The  nests  are  usually  located  in  the  woods  or  uncultivated  lands  and 
the  ants  travel  sometimes  great  distances  to  certain  trees  which  they 
choose  to  defoliate,  carrying  the  pieces  of  leaves  raised  over  their  heads. 
The  ants  look  after  this  special  fungus  with  great  care  and  are  said  to 
weed  out  all  contaminating  fungi  or  molds  and  to  nip  off  the  vegetative 
hyphae  so  as  to  cause  the  fungus  to  produce  the  particular  growth  which 
they  use  for  food.  These  ants  were  very  annoying  and  would  some- 
times completely  defoliate  several  trees  before  their  presence  would  be 
detected.  The  most  common  means  of  combating  them  appeared  to  be 
the  use  of  carbon  bisulphid,  poured  into  their  nests. 

WHITE  FLIES. 

Of  the  three  serious  citrus  white  flies  spoken  of  as  occurring  in 
Florida,  only  the  woolly  or  Cuban  white  fly,  Aleyrodes  howardii,  is 
of  any  importance  in  Cuba  and  it  is  not  considered  a  very  serious  pest. 
A  few  specimens  of  what  appeared  to  be  the  cloudy  wing  white  fly, 
Aleyrodes  nubifera,  were  seen  near  the  Cuban  Experiment  Station, 
where  it  was  not  abundant  enough  to  be  considered  a  pest.  The  woolly 
species  was  seen  in  several  localities,  but  was  not  serious.  Both  these 
species  appeared  to  be  kept  well  in  check  by  some  natural  enemies, 
perhaps  by  both  fungus  and  insect  parasites.  A  fungus  parasite, 
Aschersonia  aleyrodes,  has  been  found  on  both  these  species  in  Cuba. 

SCALE    INSECTS  AND   MITES. 

A  number  of  different  scale  insects  exist  on  citrus  trees  in  Cuba,  but 
their  injury  is  lessened  on  well-cared-for  trees  in  moist  localities  because 
of  the  prevalence  of  a  number  of  fungi  that  are  parasitic  upon  them. 
As  in  Florida,  the  most  widely  distributed  scale  insect  appeared  to  be 
the  purple  scale,  Lepidosaphes  beckii.  It  is  attacked  also  as  in  Florida 
by  the  white,  red  and  black  fungi,  known  scientifically  as  Ophionectria 
coccicola,  Sphcerostilbe  coccopliila,  and  Myriangium  duricei. 


200  UNIVERSITY   OF    CALIFORNIA — EXPERIMENT   STATION. 

Next  to  the  purple  scale,  perhaps  the  small  white  Chionapsis  scale 
(Chionaspsis  citri)  was  the  most  important.  It  is  common  especially  on 
the  limbs  of  old  neglected  citrus  trees  and  although  attacked  by  some  of 
the  parasitic  fungi,  is  not  so  fully  kept  in  check  as  some  others.  Myri- 
angium  duricei  and  Sphcerostilbe  coccophila  were  found  attacking  it. 

Perhaps  the  third  scale  insect  in  importance  was  the  large  turtle  back 
scale,  Lecanium  sp.  W.  T.  Home  in  a  letter  says  of  it :  "It  is  doubtless 
one  of  the  three  worst  orange  scales  in  Cuba.  It  is  very  bad,  has  a  vile 
odor  and  goes  down  on  the  roots.  It  is  also  on  avocado.  It  is  bad  in 
dry  seasons,  but  cheeked  by  its  enemies  in  wet  seasons." 

The  other  scale  insects  more  or  less  common  were  Chrysomphalus 
aonidum,  Lepidosaphes  gloverii,  Parlatoria  pergandei,  Coccus  hesperi- 
dum,  Ceroplastes  floridensis. 

Mealy  bugs,  red  spiders  and  rust  mites  appeared  to  be  nearly  the 
same  in  importance  as  in  Florida.  For  a  more  complete  description  of 
insect  pests  of  Cuba,  see  Bulletin  9,  of  the  Cuban  Experiment  Station, 
by  Cook  and  Home. 

DISEASES  OTHER  THAN  INSECT  PESTS  IN  CUBA. 

Since  many  of  the  diseases  have  been  discussed  at  some  length  in 
connection  with  Florida  conditions,  they  will  be  little  more  than  men- 
tioned in  this  discussion  of  Cuban  diseases.  Of  diseases  other  than 
insects  perhaps  the  various  forms  of  gum  diseases  were  the  most  im- 
portant. The  different  forms  of  gum  disease  were  practically  the  same 
as  those  of  Florida,  except  that  Florida  scaly  bark  or  nail-head  rust  was 
not  found  there. 

GUM    DISEASES. 

Mai  di  gomma  or  "foot  rot,"  as  it  typically  occurs  in  Florida  on 
old  seedling  orange  trees,  and  a  few  cases  of  Psorosis  (California 
scaly  bark)  were  seen.  Diplodia  gumming,  due  to  Diplodia  natalensis, 
in  which  fair  sized  branches  were  killed  back  was  probably  the  most 
common  of  the  gum  diseases.  It  was  seen  in  nearly  every  locality 
visited.  This  may  possibly  be  the  disease  spoken  of  under  "twig  gum- 
mosis"  by  W.  T.  Home,  in  Bulletin  9  of  the  Cuban  Experiment  Station. 
Grapefruit  (pomelo)  trees  seemed  especially  susceptible  to  this  trouble. 
The  same  fungus  was  also  found  in  nearly  every  locality  in  decaying 
pomelo  fruits.  A  gum  disease  on  the  trunks  of  lemon  trees  was  also 
fairly  common,  but  it  appeared  to  take  a  different  form  from  either  the 
Pythiacystis  or  Botrytis  gummosis  in  California  or  foot  rot  in  Florida. 
There  appeared  to  be  a  large  amount  of  exudation  of  gum  with  a  small 
amount  of  killing  of  bark.  The  outer  bark  was  killed  ahead  of  the 
inner,  somewhat  like  the  Botrytis  form,  but  the  Botrytis  fungus  was 
not  found  in  it. 


[Bulletin  262]  citrus  diseases.  201 

The  most  serious  form  of  gum  disease  in  Cuba,  common  on  heavy 
soils,  appeared  to  be  the  Pythiacystis  gummosis.  Beginning  at  the 
base  of  the  trunk  the  bark  was  killed  for  some  distance  above  the  ground 
with  the  exudation  of  considerable  gum.  This  form  in  most  cases  resem- 
bled closely  the  Pythiacystis  (brown  rot)  gummosis,  as  it  occurs  in  Cali- 
fornia on  heavy  soils.  It  is  strongly  suspected  that  this  may  be  only 
one  form  of  mal  di  gomma  (see  previous  discussion  under  California  and 
Florida).  The  bark  was  killed  rapidly  clear  through  to  the  wood  and 
had  the  characteristic  odor  of  bark  killed  by  Pythiacystis.  From 
specimens  of  such  bark  from  a  grapefruit  grove  at  Santiago  de  las  Vegas, 
cultures  of  Pythiacystis  citrophthora  were  obtained.  As  a  culture  of 
Pythiacystis  was  also  obtained  from  similar  specimens  on  the  Isle  of 
Pines  and  also  at  Palmetto,  Florida,  it  would  seem  that  this  fungus  is 
the  causal  agent  in  that  particular  type  of  gummosis  there  as  well  as 
in  California. 

Scab  (Verrucosis)  Cladosporium  citri.  Probably  next  in  importance 
to  the  gum  diseases,  was  scab,  especially  on  grapefruit.  During  years 
when  the  moisture  and  growth  conditions  are  favorable  for  infection  by 
the  scab  fungus,  a  great  deal  of  trouble  is  experienced  from  this  disease. 
It  attacks  the  young  fruits,  just  as  they  are  growing  most  rapidly.  The 
off  bloom  or  "  June  bloom"  fruit  is  most  apt  to  be  infected  seriously. 
The  leaves  also  may  be  contorted  and  warped  out  of  shape  by  it.  The 
copper  sprays  (Bordeaux  and  ammoniacal  copper  carbonate)  have  been 
used  successfully  in  preventing  it,  but  the  increase  of  scale  insects  due 
to  killing  off  of  the  fungus  parasites  of  the  insects  is  so  rapid  that  many 
growers  hesitate  to  spray  with  these  copper  fungicides.  Lime  sulfur  is 
being  tried  out  and  is  reported  to  have  given  good  results. 

Wither-tip  of  Limes  (Glceosporium  Lemitticolum.  "Wither-tip  of 
lime  trees,  the  same  as  occurs  in  south  Florida,  appeared  to  be  a  prev- 
alent and  serious  disease  throughout  the  island.  It  blights  the  new 
growth  as  it  comes  out  and  interferes  materially  with  the  setting  of 
young  lime  fruits.  This  is  the  disease  which  Mr.  Roy  Clausen  of  the 
University  of  California,  determined  to  be  due  to  a  fungus  resembling 
but  apparently  quite  distinct  from  the  Collet otrichum  glozosporioides, 
which  is  associated  with  wither-tip  of  other  varieties  of  citrus  in  both 
Florida  and  California. 

Blossom  End  Rot  of  Persian  Limes.  Wherever  the  Persian  limes 
were  found,  there  was  a  certain  amount  of  a  firm  rot  at  the  "  blossom " 
or  stylar  end  of  the  fruits.  This  appears  to  be  the  same  as  has  been 
observed  in  Florida  and  in  one  case  in  California.  It  appears  first  as 
a  whitish,  sunken  patch  around  the  stylar  end.  This  patch  remains 
firm  and  does  not,  as  a  rule,  soften  much  as  in  some  other  kinds  of 
decay.     The  cause  is  as  yet  unknown.     It  would  appear  to  be  due  to  an 


202  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION. 

organism  of  some  kind.  A  fungus  resembling  Collet otrichum  glceos- 
porioides  was  found  commonly  in  the  spots,  but  was  not  determined  to 
be  the  cause  of  the  trouble. 

Diplodia  Rot  (Diplodia  natalensis).  A  decay  especially  of  grape- 
fruit which  begins  at  the  stem  end  and  slowly  rots  the  fruit,  was  quite 
common.  (The  "stem-end  rot,"  due  to  Phomopsis  citri  was  not  seen). 
Cultures  made  from  such  fruits  at  various  localities  showed  the  Dip- 
lodia fungus  to  be  present.  This  aside  from  the  blue  molds  appeared 
to  be  the  most  important  decay.  The  same  fungus  has  already  been 
mentioned  in  connection  with  the  Diplodia  gumming  on  branches.  In 
some  places  considerable  tear  streaking,  probably  due  to  the  wither-tip 
fungus,  was  also  seen. 

Leaf  Spot,  due  to  an  Alga  (Mycoidea  parasitica) .  A  certain  amount 
of  damage  appears  to  be  due  to  a  species  of  alga  attacking  the  leaves, 
producing  a  wart-like  growth.  This  is  not  known  to  be  present  on  citrus 
in  either  Florida  or  California.  The  damage  from  any  one  spot  is 
negligible,  but  when  the  leaves  become  covered  with  spots  they  are 
weakened  and  fall  off.  It  has  not  usually  been  considered  serious 
enough  to  use  any  methods  of  prevention  in  Cuba.  Any  fungicide 
would  probably  prevent  it.36 

Dark  Greasy  Spot  ("Black  Melanose").  Although  the  true  Mel- 
anose  appeared  to  be  absent  in  Cuba  and  the  Isle  of  Pines,  a  dark,  greasy 
spot  sometimes  called  in  Florida  "Black  Melanose,"  was  very  common, 
especially  on  grapefruit  leaves.  The  writer's  observations  would  lead 
him  to  say  that  this  dark,  greasy  spot  increases  in  prevalence  and  effect, 
as  one  passes  from  the  northern  to  the  southern  part  of  Florida  and  is 
even  more  prevalent  in  Cuba  and  the  Isle  of  Pines.  The  spots  have  a 
dark,  slightly  raised,  greasy  appearance,  suggestive  of  a  mass  of  dark 
grease  under  a  transparent  epidermis.  Their  development  on  leaves 
in  Cuba  is  much  more  definite  than  in  most  parts  of  Florida  and  would 
strongly  suggest  the  effect  of  some  organism.  This  would  be  suspected 
even  more  strongly  since  the  discovery  by  H.  E.  Stevens  in  Florida,  that 
the  true  Melanose  is  due  to  Phomopsis  citri,  the  same  fungus  as  causes 
stem-end  rot. 


"Cook  and  Home.     Cuban  Agri.  Exp.   Sta.   Bui.   9. 


[Bulletin  262] 


CITRUS    DISEASES. 


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STATION  PUBLICATIONS  AVAILABLE  FOR  DISTRIBUTION. 


REPORTS. 

1897.    Resistant   Vines,    their    Selection,    Adaptation,    and    Grafting.    Appendix    to    Viticultural 
Report  for  1899. 

1902.  Report  of  the  Agricultural  Experiment  Station  for  1898^1901. 

1903.  Report  of  the  Agricultural  Experiment  Station  for  1901-03. 

1904.  Twenty-second  Report  of  the  Agricultural  Experiment  Station  for  1903-04. 

1914.  Report  of   the   College  of   Agriculture   and  the  Agricultural   Experiment   Station,    July, 

1913- June,  1914. 

1915.  Report   of   the   College  of   Agriculture   and   the  Agricultural   Experiment   Station,    July, 

1914-June,   1915. 


No. 
168. 

169. 
170. 
174. 
178. 
184. 

185. 

195. 
197. 


203. 

207. 
208. 
211. 

212. 
213. 

216. 


220. 
225. 


Observations    on    Some    Vine    Diseases 

in  Sonoma  County. 
Tolerance  of  the  Sugar  Beet  for  Alkali. 
Studies  in  Grasshopper  Control. 
A  New  Wine-Cooling  Machine. 
Mosquito   Control. 
Report    of    the    Plant    Pathologist    to 

July  1,  1906. 
Report   of   Progress   in   Cereal  Investi- 
gations. 
The  California  Grape  Root-worm. 
Grape  Culture  in  California;  Improved 

Methods      of      Wine-making;      Yeast 

from   California   Grapes. 
The  Grape  Leaf-Hopper. 
Report    of    the    Plant    Pathologist    to 

July  1,   1909. 
The  Control  of  the  Argentine  Ant. 
The  Late  Blight  of  Celery. 
How  to   Increase  the  Yield   of   Wheat 

in  California. 
California  White  Wheats. 
The  Principles  of  Wine-making. 
A     Progress     Report     upon     Soil     and 

Climatic      Factors      Influencing      the 

Composition  of  Wheat. 
Dosage  Tables. 
Tolerance   of   Eucalyptus   for  Alkali. 


BULLETINS. 

No. 

227. 


234. 
241. 
242. 

244. 
246. 
248. 

249. 
250. 
251. 


252. 
253. 

254. 

255. 
256. 

257. 
258. 
259. 

260. 

261. 


Grape  Vinegar. 

Enological  Investigations. 

Red  Spiders  and  Mites  of  Citrus  Trees. 

Vine  Pruning  in  California.    Part  I. 

Humus  in  California  Soils. 

Utilization  of  Waste  Oranges. 

Vine  Pruning  in   California.    Part   II. 

The  Economic  Value  of  Pacific  Coast 

Kelps. 
Stock  Poisoning  Plants  of  California. 
The  Loquat. 

Utilization    of    the    Nitrogen    and    Or- 
ganic   Matter    in   Septic    and    ImhofY 

Tank  Sludges. 
Deterioration  of  Lumber. 
Irrigation   and   Soil   Conditions   in   the 

Sierra   Nevada   Foothills,    California. 
The  Avocado  in  California. 
The  Citricola  Scale. 
The    Value    of    Barley    for    Cows    fed 

Alfalfa. 
New  Dosage  Tables. 
Mealy  Bugs  of  Citrus  Trees. 
Commercial  Fertilizers. 
Availability     Studies     on     Nitrogenous 

Fertilizers. 
Melaxuma    of    the    Walnut,    "Juglans 

regia." 


No. 


70. 


82. 


87. 

91. 
100, 
101, 

106, 

107, 

108. 
109, 


110, 
111. 

113. 
114. 
115. 
117, 

118. 
119. 


The  California  Insecticide  Law. 

The   Extermination   of   Morning-Glory. 

Observations  on  the  Status  of  Corn 
Growing  in  California. 

Hot  Room  Callusing. 

List  of  Insecticide  Dealers. 

Boys'  and  Girls'  Clubs. 

The  Common  Ground  Squirrels  of 
California. 

Potato  Growing  Clubs. 

Alfalfa. 

Disinfection  on  the  Farm. 

Pruning  Frosted  Citrus  Trees. 

Codling  Moth  Control  in  the  Sacra- 
mento Valley. 

Directions  for  using  Anti-Hog  Cholera 
Serum. 

Spraying  Walnut  Trees  for  Blight  and 
Aphis  Control. 

Grape  Juice. 

Community  or  Local  Extension  Work 
by  the  High  School  Agricultural  De- 
partment. 

Green  Manuring  in  California. 

The  Use  of  Lime  and  Gypsum  on  Cali- 
fornia Soils. 

Correspondence  Courses  in  Agriculture. 

Increasing  the  Duty  of  Water. 

Grafting  Vinifera  Vineyards. 

The  Selection  and  Cost  of  a  Small 
Pumping  Plant. 

The  County  Farm  Bureau. 

Winery  Directions. 


CIRCULARS. 

No 
121. 


122. 

124. 
125. 
126. 
127. 
128. 
129. 
130. 
131. 

132. 

133. 
134. 
135. 
136. 

137. 
138. 
139. 


140. 


141. 
142. 


Some  Things  the  Prospective  Settler 
Should  Know. 

The  Management  of  Strawberry  Soils 
in  Pajaro  Valley. 

Alfalfa  Silage  for  Fattening  Steers. 

Aphids  on  Grain  and  Cantaloupes. 

Spraying  for  the  Grape  Leaf-Hopper. 

House  Fumigation. 

Insecticide  Formulas. 

The  Control  of  Citrus  Insects. 

Cabbage  Growing  in  California. 

Spraying  for  the  Control  of  the  Wal- 
nut Aphis. 

When  to  Vaccinate  against  Hog 
Cholera. 

The  County  Farm  Adviser. 

Control  of  Raisin  Insects. 

Official  Tests  of  Dairy  Cows. 

Melilotus  Indica  as  a  Green  Manure 
Crop  in  Southern  California. 

Wood  Decay  in  Orchard  Trees. 

The  Silo  in  California  Agriculture. 

The  Generation  of  Hydrocyanic  Acid 
Gas  in  Fumigation  by  Portable  Ma- 
chines. 

The  Practical  Application  of  Improved 
Methods  of  Fermentation  in  Califor- 
nia Wineries  during  1913  and  1914. 

Standard  Insecticides  and  Fungicides 
versus  Secret  Preparations. 

Practical  and  Inexpensive  Poultry 
Appliances. 


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